I'm not sure what method you are using to back up data as you have not stated explicitly what you are doing. I have recently come across some people still using Mozbackup - I strongly advise you do not use that program as it has not been updated in over 12 years and even the author advises it has bugs and use at own risk.

The following is a simple and quick method of backing up everything.

I'm not sure whether you have imap or pop mail accounts but either way you have to ensure you have downloaded full copies of all emails in all folders. There is no point in backing up just headers, so check you have fully downloaded good copies of emails. Pop should have fully downloaded emails -

• Account Settings > Server Settings
• If you selected 'Fetch Headers only', then you have not downloaded full copies.
• Uncheck that option and then clickon the 'Get Messages' and see if you can get full downloaded copies from server.

Imap -

• Account Settings > Synchronisation & Storage.
• Select checkbox - 'keep messages in all folders for this computer'
• clickon 'Advanced..' button
• Select checkboxes for every folder and subfolder to get fully downloaded copies and clickon OK
• Under 'Disk Space'
• select 'Synchronise all messages locally regardless of age'

The force a synchronisation via 'Menu Bar' toolbar

• File > Offline > Download sync now
• Select 'Mail Messages'
• You can double check all required folders are selected using the 'Select...' button
• Click on OK's to start the sync.

I would also advise you compact all folders to remove all the old traces of moved or deleted emails. So you are backing up only wanted emails in the least size files.

• In Folder Pane
• select name of account folder and right click to see drop down menu and clickon 'Compact Folders'
• REpeat for each mail account name folder.

Now you ready to form a backup.

• Exit Thunderbird now - this is important.

Now perform the backup... Access where the profile is located - I'm assuming it is in the default Windows OS location.

• In windows search type: %Appdata% - see image below as guide.
• Select the %Appdata% file folder
• select the 'Roaming' folder
• Right click on the 'Thunderbird' folder and select 'Copy'
• Access external hardrive or where ever you put backup
• Right click and select 'Paste'

The 'Thunderbird' folder contains absolutely everything required should it be required as a backup. That includes 'profile.ini' files, profile name folders and files therein including emails, address books, calendar and all preferences including passwords etc. It also means it's easy to put a complete copy back on computer should you need to.

By the way Thunderbird works for any server that offers pop, imap and smtp access for emails. In some cases, the server requires that you need to switch on the options you want to use. Xfinity states you need to select a specific checkbox option to allow third part access. Some servers now require you to use 'app specific passwords' eg: yahoo - many email addresses use the yahoo server. Other servers like gmail and microsoft require you to use 'Authentication Method: Oauth2' for both receiving and sending and will prompt you to enter password in order to allow third party access to server.

REcently, Microsoft has changed a few more rules, Microsoft account password must be the same as outlook.com/hotmail.com/live.com/msn.com account/email address password. Microsoft has also now stated only one account will be classed as the main account - set up as 'primary' and any other email addess eg: outlook.com/hotmail.com/live.com/msn.com will be regarded as 'alias' account. This has an impact on sending using third party programs - no matter what the account email address used for receiving emails, it must use whatever is set up as 'primary' as the SMTP username. However, you do have the choice on what you choose to be as 'primary'.

I notice in a previous question you said - GMX does not work with Thunderbird It does, but as is the case for any email address on any server - obviously you do have to use very specific settings. Get any part wrong and it will fail.

I notice you also said : I read on THIS FORUM that Gmail did not work well with Thunderbird Not true, it works perfectly well. Thunderbird has no problem providing you use correct settings. In fact there has been huge progress recently when it comes to gmail. Now you easily hookup to the gmail Address Book and Calendar.

Info you may not be aware about:

When you attach any file(s) to an email you will increase the email size, which you probably already realise, but what you may not realise is the following: When email is prepared for sending the size increases by about a third due to base64 encoding sending two 8-bit characters as three 7-bit ASCII characters.

So, if you attach a file that is eg: 18 MB, then it real size will be close to 25MB and may fail because either your server will not allow sending of an email of that size or the receiving server cannot receive a file of that size. You have to consider not just whether your server will send it but whether the receiving server will accept it.

Advise you make sure any email you send has a total size not exceeding 16 MB, then you know it will not exceed 25MB.

Or less than 14MB, if the max is 20MB.

You could zip the file to make it smaller. Or if an image reduce the dpi or size to make smaller file.

Please note: In older version the word 'Options' was used - this was then replaced by the word 'Preferences' but in the most up to date recent version of Thunderbird is now replaced by 'Settings'.

Read this info on sending large files as attachments

• https://support.mozilla.org/en-US/kb/how-can-i-send-large-attachment

Info on FileLink for large attachments:

• https://support.mozilla.org/en-US/kb/filelink-large-attachments

FileLink is under 'Settings' > 'Composition' Scroll down to 'Attachments' which is at the bottom.