Hello richie5

you most probably have an antivirus that is interfering with the network connection between your mail client (Thunderbird) and your server. You could confirm that by starting Windows in safe mode with networking and test again. If it solves your problem, you should setup your antivirus to disable realtime scanning of network connections. Here is not the best place to find directions on how to do that, your antivirus community or support (Avast forum probably...) is.

Hi gp, thanks for the suggestion, but that didn't help. Do you think it has anything to do with me still using Windows 7, not being fully compatible with the new upgrade..?

It is very unlikely that Windows 7 is the culprit. An antivirus is most often responsible, but sometimes ther ecan be a firewall integrated in the Internet box (or not). Also try to raise the timeout for smtp; mailnews.tcptimeout in the config editor (settings / general scroll all the way down to find the button 'config editor', enter the parameter, set a higher value for example 300.

Just set it to 300, still timing out before it gets to 40%

It is very unlikely that Windows 7 is the culprit. An antivirus is most often responsible, but sometimes ther ecan be a firewall integrated in the Internet box (or not). Also try to raise the timeout for smtp; mailnews.tcptimeout in the config editor (settings / general scroll all the way down to find the button 'config editor', enter the parameter, set a higher value for example 300.

I changed the value up to 800 and it still times out. Funny thing is, on my phone using Thunderbird, I can send emails with attachments, but not from my PC.

As far as I know, 'Thunderbird' for Android is really K9-Mail, a completely different software. And there is no firewall on Android (I still think that the most likely culprit is a firewall). Take a look at a post where I outlined all I could think of about a similar problem:

https://support.mozilla.org/en-US/questions/1398038#answer-1550820