I have a similar problem. I use Zen Internet as ISP. Using TB, I can send emails over WiFi from home, but nowhere else. I can not send over mobile data anywhere. Proton Mail, Gmail and Outlook do not have these problems. K9 does. The message is SmtpResponseparser exception. Unexpected end of Stream. TB says my ISP settings are OK.

I have this problem, too, only there is no sign that anything is wrong except no one gets my messages! Well, my sister, it's her domain. I guess I should've said my messages aren't going outside the domain.

Also, the page of what to do if your messages aren't going out is useless. I go were it says to go and what it says should be there isn't.

My problem is same as @khindall's. No one receives my emails sent from Android TB. No error or bounce messages. Emails are in Sent folder. Desktop version works fine.

Me too! Most messages I try to send end up in the outbox but I can't find a way to get them sent. Some messages get sent but probably less than 10%.

In my Outgoing Server settings, everything looks the same as on my Windows P.C., which works fine, except Client Certificate is a field that I don't find on my P.C. It is set to None on my phone.