If you change the outgoing server to smtp.office365.com on port 587, connection security = STARTTLS, authentication = normal password, and User name = email address, does it make any difference?

Thanks for the reply.

I sent an email to myself to test that, but while I was able to send it, I wasn't able to receive it until I changed the server settings back.

I got the following error messages:

There was an error saving the message to Sent. Retry? [right after sending]

and

Mail server XXXXXXXXXX@hotmail.com is not an IMAP4 mail server.

I have two accounts:

First is a hotmail account, and the second is an outlook account run by my college.

Both are simultaneously intermittently unable to send email sometimes. I have as of yet to determine a pattern, and restarting my laptop doesn't fix it.

The error message on sending may occur if you have 'Place a copy in:' checked in Account Settings under Copies & Folders, as hotmail/live/outlook accounts automatically copy sent messages to the Sent folder.

The correct settings for hotmail/live/outlook accounts are:

incoming IMAP server: outlook.office365.com port: 993 Connection security: SSL/TLS Authentication: normal password User name: email address

Outgoing SMTP server settings as mentioned in my first reply. If you've set up 2-step verification on the account, create an app password. If you're changing the settings on an existing account, remove passwords from Tools/Options/Security/Passwords/Saved Passwords, restart TB, and enter the correct password when prompted.

Okay, I don't know what I was changing before with the outgoing server, but it wasn't the right setting.

I did finally find the outgoing server settings, and they have been as you described in your first post.

Is there any way to trouble shoot/determine what the cause is while it's not able to send emails? Is there a log file?

You should get an error message if sending fails, although even then the message sometimes doesn't reveal the source of the problem. Antivirus/firewall programs are a common source, besides the obvious one of incorrect settings.

I have bitdefender for Mac, and it doesn't have a white list option that I know of for blocking application access to networks, I'll ask them about it.

Thanks for your replies.

The usual method is to set the AV program to exclude the TB profile folder from the resident or background scanning. An example for Bitdefender is given here. The profile folder is easily found from Help/Troubleshooting Information and clicking Open Folder in the Application Basics section, or by clicking about:profiles and noting the root directory.

The firewall component of your security software is probably not blocking TB from network access, but in some cases it needs to be 'reminded' when TB is updated.

I wasn't aware that setting would whitelist the app from network blocking. I thought it would only remove it from the files it would scan for a system virus scan. You see, I'm on a macbook, and Bitdefender for Mac isn't as fleshed out as of yet as the PC version, and that option seems to be merged with another.

So far, I sent several emails, and no trouble so far.

Thanks for your help!