This is a complex issue that really have very little to recommend it as far as a "do this it works" cure. Try disabling any scanning of outgoing mail in your virus package as this can have a serious impact. For that matter try without any internet scanning as all as some antivirus scanners consider mail ports to be something they scan, even if mail is specifically excluded as a service.

Most folk with GMail and outlook.com have the issue because the provider is updating the sent folder with their copy of the sent mail at the same time Thunderbird is also trying to do it. Their solution is to turn off Thunderbird making a copy at all.

For others it is virus scanning that slows the process enough for it to timeout. Yet others are in a situation that the upload bandwidth is simply saturated. Multiple users in the house all uploading things at the same time (think snap chat for a constant stream of large uploads) fundamentally the issue is mostly around the connection trying to upload the message times out. This should generally be worse of large email than small ones.

Most consumer internet is significantly faster downstream than upstream. Like 4 times faster, the assumption is that consumers consume content, not make it. It is also a way to sort of force commercial operators that might want to run a business using the internet to buy a commercial symmetric package (the primary difference between consumer and business packages)

I looked at the windstream settings which they describe as https://www.windstream.com/support/my-account/email-101/server-settings-for-windstream-email

I strongly suggest you confirm the settings for outgoing server SMTP. YOu might also find that an unencrypted connection (no SSL) causes more of less issues. This would demand on a number of things, not the least of all the amount of data being send and received is less with unencrypted connections. Not much different but it is a difference there.

Thank you, Matt. I read your reply, but am too tired to try any of your suggestions tonight. They may be beyond my expertise. I've used computers since 1985, but that means that I am not young. We use the free version of Avast. I really do appreciate your taking the time to offer help!

It's worth checking the Avast Anti-Virus settings first. Switch these two settings off and leave switched off for a decent amount of time eg: at least a week, so you get a good idea of whether this improves the sending and saving of emails.

Try stopping the scanning of all outgoing mail: It's found under Shields -> Mail Shield -> Settings. Disable "Scan Outbound Mail". Good link: https://support.avast.com/en-gb/article/167/#pc Scroll down to 'Manage individual shield settings' section click on the 'Mail Shield' option on the right side.

Also check to see if Avast is also adding it's own piece of advertising to outgoing mail. Good link to avast forum: https://support.avast.com/en-gb/article/antivirus-email-signature/#pc

Thank you, Toad Hall. I have switched off the two Avast settings that you suggested. I am hoping against hope that this will work. It took me a long, long time to find the mail shield in Avast, but I finally found exactly what you described. Now, I wait and see if this solves the problem.

Many,many thanks to Toad Hall! I changed the two settings in Avast that he suggested and have had no email failures for five days. Windstream is not a fabulous ISP, but it was innocent in this case. I am delighted to have this solved after years of frustration.