I have been running Tbird on my desktop for years, and it's still working great. About 2 years ago I picked up a One Plus 11G which came with no e-mail client. I searched for Tbird and found K9 mail (with promises of Tbird soon), which I installed and used since then with no problems. Few months ago, i got my wife a Samsung23+ and she used the supplied e-mail and messaging programs but constantly complained that she could have only one notification sound for both mails and texts and wanted to tell them apart. I told her I'd have to install a separate mail app so she could have two different sounds. OK, I checked and finally found that Tbird was now available for Android so i installed it on her phone and she is very happy now. Since it worked for her, I decided to install it on my phone to replace K9 Mail, and that's when the fun started. It works great for incoming mails, but when I try to send mails via WiFi or via cell, I get a "Failed to send some messages sender refused" notification. I got another phone and installed Tbird and it behaves the same way on that phone whether I set it up as POP (my usual setting) or IMAP. I've installed other mail apps (GMAIL and Edison)and they send out with no problems both POP and IMAP. If I go to the the Optonline web mail site via Firefox on my phone, I have no issues sending out mails, but that is a clunky way to do since Tbird normally worked so well as it is now or in iots earlier K9 version. Any idea what is causing this problem only with Tbird? I really like Tbird and I don't like the other two apps.