Ok from reading an old thread where a user @IanW with Stackmail also could not send, it appears the problem was that “the server was using an outdated older CRAM-MD5 authentication protocol from the eM Client logs” so wouldn’t work with newer mail client authentication methods. So was surmised by @AffaMoh that they needed “to force the older plain text authentication” allow to then send mail.
@skybat also advised in that thread the alternative “is to use a different SMTP server”.
There was also another thread where they “also had an older different mail server authentication” and couldn’t send mail due to allways authentication verification problem, but with the below plain text settings from @Gary and @Phil_LP (it then sent mail ok) in V10.
So @jeseph12 and @IanW try putting the following in your IMAP account “Diagnostics” Tab to force the SMTP authentication to use a plain text and see if that will then allow to send mail.
(If you don’t specify the username and password try using the following in the Diag).
Or
(If you do specify the username and password in the SMTP tab, try using the following in the Diag).
Server smtp.stackmail.com
Port: 465
Security policy: Use SSL/TLS on special port (legacy)
Or
Server smtp.stackmail.com
Port 587
Security policy: Force usage of SSL/TLS
Note: @jeseph12 If trying the above strings still doesn’t allow you to send mail, then click the drop-down arrow on the right of Refresh and click Show Operations. Then click the “Log tab” and look for obvious error messages and paste them in this thread so we can check.