TB Menu Bar > File > Compact Folders. If you don't see the menu bar on top, press the ALT key.

Hi Ed, What does the Menu Bar look like and where is it? Nothing happens when I press the ALT key.

Edit Looked in Help and found the Menu Bar by clicking on a blank space,but most things under File, including Compact Folder, are greyed out.

Edit 2. So I need to select the folder first. Duhh!

Thanks. It always seems to me that things are hidden in different places on Thunderbird and you have to be an expert to find what you want

Hi Ed, What does the Menu Bar look like and where is it? Nothing happens when I press the ALT key. Edit Looked in Help and found the Menu Bar by clicking on a blank space,but most things under File, including Compact Folder, are greyed out. Edit 2. So I need to select the folder first. Duhh! Thanks. It always seems to me that things are hidden in different places on Thunderbird and you have to be an expert to find what you want

Not sure why 'Compact Folders' was grayed out; it should appear all the time. Anyway, it compacts all folders, not just the one you have selected. Did the messages stop after doing that?

Hi Ed, No, I'm still getting that message. Any more ideas?

I quite often get this message, though it tends not to be a problem later "Unable to write the email to the mailbox. Make sure the file system allows you write privileges, and you have enough disk space to copy the mailbox" One of the solutions seems to be Compact a File. How do I do this?

The usual diagnosis on this forum is anti-virus software. Are you using some? Can you create an exception for Thunderbird in it?

HI Rick. The main purpose of my antivirus is to scan emails, so turning it off doesn't seem a good idea. Also, the message comes and goes for the same email download - if I leave it, then try again, the emails download with no problem. However, the length of time to wait between attempts at downloads can be quite variable.

Also, if it is Bitdefender causing a problem, why do emails get stuck sometimes and not others?

I've just thought of a related question, but don't know if I can just add it here. Compacting email folders seems important - what size is maximal? Once you reach this limit, what do you do? In Pegasus Mail, it was simply a matter of making a new Mail folder for incoming mail after renaming the previous one

Hi Ed, No, I'm still getting that message. Any more ideas?

Can also happen if you're getting multiple copies of the same message. If you think that's happening, go to your email via the web interface and delete that message.

Still having this problem on and off - it usually appears on starting Thunderbird - if I wait a while, I can then avoid this message. But it can appear again. It's very annoying.

I posted this above, but got no answer " Compacting email folders seems important - what size is maximal? Once you reach this limit, what do you do? In Pegasus Mail, it was simply a matter of making a new Mail folder for incoming mail after renaming the previous one" Anyone any answers?

HI Rick. The main purpose of my antivirus is to scan emails, so turning it off doesn't seem a good idea. Also, the message comes and goes for the same email download - if I leave it, then try again, the emails download with no problem. However, the length of time to wait between attempts at downloads can be quite variable. Also, if it is Bitdefender causing a problem, why do emails get stuck sometimes and not others?

The problem in Thunderbird seems to be with third-party anti-virus software, not the anti-virus sofware that comes with Windows, which many people consider sufficient.

I do not know why the problem would be intermittent because I do not know exaclty what Bitdefender does. But I can imagine variability in its process of scanning incoming messages.

To solve this problem, you probably should try starting Windows in safe mode with networking (or at least disable Bitdefender, but safe mode is better), then see if the problem still occurs.

https://support.microsoft.com/en-us/windows/windows-startup-settings-1af6ec8c-4d4a-4b23-adb7-e76eef0b847f

Compacting email folders seems important - what size is maximal? Once you reach this limit, what do you do? In Pegasus Mail, it was simply a matter of making a new Mail folder for incoming mail after renaming the previous one

You'll want to have a good understanding of compacting: https://support.mozilla.org/en-US/kb/compacting-folders

The need for compacting is not due to large folder size. There's' no need to make a new folder for incoming mail and rename the previous one.

I don't know when folders should be compacted. In Settings > General > Network & Disk Sapce, I have folders set to be compacted when 50 MB of storage space can be recovered.

Very large folders may be more prone to some problems. I don't know.

I posted this above, but got no answer " Compacting email folders seems important - what size is maximal? Once you reach this limit, what do you do? In Pegasus Mail, it was simply a matter of making a new Mail folder for incoming mail after renaming the previous one" Anyone any answers?

See 'Folders and messages' here.

It's up to you when to compact folders.

HI Rick. The main purpose of my antivirus is to scan emails, so turning it off doesn't seem a good idea.

Yet these ubiquitous product are the principal cause of issues when it comes to email. Why are you scanning email? Seriously. Microsoft defender does not do that at all and still manages to get almost all of the same threats detection scores in the tests as the expensive third party products. What it does not have are all of those reassuring settings and claims of safety and the bells and whistles that only sort of work and only for a limited time. Some 15 years ago I challenged the folk at Norton (then Symantec) about what exactly was the point of having email scanning after a long back and forth discussion the best they could offer was "it is another line of defense" The only defense that can be guaranteed is total isolation physically from other electronic devices. We are not going to do that. Personally I stopped the scanning of email foolishness more than a decade ago. But lets look at the email you receive so you can understand my position on that.

Email as it comes into your computer is a plain text file. Completely inert and unable to do anything infect anything or cause any issues at all. Thunderbird even blocks remote images (that is a privacy thing more than a security thing) Thunderbird interprets the HTML content and displays the pretty formatting, because that is what folk claim to like. It does sanitize the HTML and some HTML tags are totally ignored as a security risk like iFrames, but plain text email is a more secure option, yet basically no one uses it, including me.

Folk always talk about attachments like they are something different, a file even. They are not. One of the "failings" that goes back to the mid 1980s of email is that for transmission everything in the body of the email, including attachments, must be rendered as plain text. That office document! Encoded using MIME to convert it to text at the point of attachment. That Zip file, also converted to text. You can not do anything with it at all until the file is actually converted back from text into it's original binary representation so when the antivirus is scanning it, it decodes it, makes it into a file and scans it. It can not scan the MIME text for a threat signature. Thunderbird only does that when you choose to open the attachment of save it. Until then it is nothing but a glorified illusion of a file. My contention is scanning on download is actually no safer than scanning the content when you are ready to do something with it.

The decoding from MIME occurs in memory and results in a temporary file in the system temp folder. Any antivirus worth being installed is actively monitoring both memory and the temp folder for threats. So if your attachment is a threat it will be identified and stopped before it can be opened. The exact same process as occurs when the email is "scanned" but delayed. If you just delete the email when you get it then the attachment never exists on your computer unless you allow an antivirus scanner to scan incoming email.

So hopefully I have explained myself in questioning importance of scanning and not just irritated you. There is also a setting in Thunderbird that can be enabled that causes Thunderbird to write each new mail to the temp folder before it incorporates it, so the antivirus can scan the content at that point without getting involved in the actual download if you really want email scanning . The scanning of incoming mail as it comes over the wire is problematical at best as the antivirus products hack the encrypted connections used to retrieve mail, again reducing security to scan something they do not really need to scan at that point.

You mention bitdefender and it has many many paid reviews that says it is one of the best. I gave up documenting the disastrous standards of antivirus products long ago. It was taking more time than it was worth to me. But have a look here. Someone is still updating some of the issues. https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#Bitdefender

You will see that the bitdefender antispam tool is mentioned. That tool causes Thunderbird to crash and stop responding after almost every update. It your antivirus (any antivirus, not just bitdefender) has a button or a toolbar in your mail client, get an antivirus that does not. They are all only good for a single version of Thunderbird and with monthly updates now they will probably make your life hell, until you move to webmail or change to a less intrusive antivirus. 10 or 15 years ago Thunderbird's calendar was only good for a single release like that it required what was called binary compatibility. The developers finally incorporated it so there was not an update required with every Thunderbird update and a huge support issue went away overnight. No one is fixing the antivirus products until after they fail. so just avoid those with feature that are likely to fail is my feeling.

Folder sizes are not an issue in Thunderbird. The upper limit is what your operating system will support on that particular file system and your available RAM. I have a number of folders over 8Gb in size and some with well over 150,000 emails in them. Compacting as Ed says is not really about saving space. The reality is deleted mail is not truly deleted until a compact removes the actual data from the folder (in an IMAP account it Expunges on a pop or local folder it simple rewrites the file without the deleted mails data. Much like old fat file systems just overwrote the first character of a file name and it was hidden, Thunderbird changes the status of the email to deleted and it is hidden. Like with the file system deleted is not actually deleted until it is overwritten. You can see this in action by changing the "what to do" in sever setting for deletion to "just mark it as deleted" and all your deleted mail will appear in the folder with a strikethough until a compact finally removes it

I do not use any antivirus now but Defender, and an occasional online scan just for peace of mind. Those that have antivirus really need to stop their antivirus product scanning Thunderbird files while it is trying to use them. File contention is a real issue when your mail folders get large and scanning takes longer than the time that elapses between checks for new mail or accesses. In this instance how long does it take bit-defender the best I can come up with is this tpic https://community.bitdefender.com/en/discussion/100037/how-long-does-a-full-system-scan-take?tab=accepted to scan a changed 5Gb file (the Gmail all mail folder for most folk is more than 4Gb these days.)

Now think Thunderbird might open and edit that file every few seconds, first once for each new mail in the folder, again when you start reading the emails and their status changes from read to unread. Again when you move it to archive or delete it. So potentially that file might be opened and changes four or five times in a matter of a minute of so for one new email. In the background this antivirus product is lumbering on. Scanning plain text files like as if they contain some immediate threat. I have no idea if it restarts it's scan multiple times or just keeps plugging away. What I do know it not having those files under constant scanning can only be a benefit, both to system speed and data integrity. Nothing works well when two things are trying to access the same resource at the same time.

And while it is not apparent in this discussion, there is a compact on the right click menu of just about every folder in the folder pane. Some virtual folders obviously do not exist so do not have a compact option.

Compacting is but one of the suggested solution as it tends to clear file corruption as it re-downloads IMAP folders header to verify the integrity of the folder and rewrites the file for POP and other local folders. IT also makes the f9ile smaller, so the other processes complete against it faster. It is rarely a raging success as these days the problem is rarely data corruption as most folk have SSDs and they are not as prone to bad sectors as the old spinning disks. But the smaller files can help sometimes if the folder size is marginal.

Other issues are;

• the most critical is file contention. This can have many causes, we tend to jump to antivirus products, because they are everywhere and are of questionable code quality. (they update them every hour or so. Who is checking the code that gets updated that often? another hallucinating AI perhaps)
• streaming backups, storage of the profile on a NAS or other network storage can cause contention issues as well.
• Placing Thunderbird's profile in one of the "archive to the cloud locations" can also cause issues as the files get backed up continuously, not once a day say. So it is a bit like the antivirus stuff I mentioned earlier. It might start backup up a 5gb file over and over causing Thunderbird to be unable to open it for writing.
• Then there is that old cottage industry of placing Thunderbird profiles into documents to facilitate something, generally I am told backup, which results in the mail store being treated as a document not application data. Folk that do that are often those with problems.

Am currently working through these options Yes, Bitdefender can interfere with Mozilla Thunderbird’s ability to read or write to email accounts, frequently causing issues with sending, receiving, or syncing messages. These issues are typically caused by Bitdefender’s security modules, such as encrypted web scanning, firewall rules, or antispam filters, incorrectly flagging Thunderbird traffic. Common Issues Caused by Bitdefender: Mail Corruption/Sync Issues: Bitdefender (specifically "Encrypted web scan") has been known to cause incoming email content to appear corrupted or blank. Failed Sending/Receiving: Bitdefender may block SMTP (sending) or POP3/IMAP (receiving) connections, especially after an update. Unrecognized Certificate Warnings: The Anti-Spam module can cause Thunderbird to throw security warnings regarding server certificates. False Positives: Bitdefender may flag thunderbird.exe as a vulnerability exploit, preventing the application from opening or operating. How to Fix or Work Around the Issues: If you are experiencing these issues, try the following steps: Disable Encrypted Web Scan: Go to Bitdefender > Protection > Online Threat Prevention > Settings and turn off Encrypted web scan. Add Exceptions: Add thunderbird.exe to the exclusions list in Bitdefender's Advanced Threat Defense and Online Threat Prevention settings. Exclude Profile Folder: Exclude your Thunderbird profile folder from real-time scanning to prevent mail file corruption. Configure Firewall: Ensure the Bitdefender firewall is not set to "Block" for Thunderbird, or set it to "Automatic" to allow appropriate rules. Disable Anti-Spam: If necessary, turn off the Bitdefender Antispam toolbar extension within Thunderbird. For a quick test, you can temporarily disable Bitdefender to confirm it is the source of the issue, but it is highly recommended to use the exceptions or configuration adjustments listed above for long-term use.