I have created an alias or better identity for my primary mail account, which uses my own domain. The alias however, is a Gmail address. I forward all my incoming Gmail mails to my primary address, so that I receive all mails with that one account.
When I now answer to such a forwarded mail, K9 choose the correct alias. However the mail will be sent via the standard SMTP of the primary address and not the Gmail SMTP server. How can I setup the SMTP server for the alias?
That’s not supported right now. It’s something we want to add, but it’s currently not a very high priority.
As a workaround you could add the Gmail account and configure it to never check for new messages. Then you could use the initial identity of that account to send messages. It has a couple of drawbacks, though. One is that sent messages will be uploaded to the Sent folder of the Gmail account (you probably want to disable that in K-9 Mail; Gmail automatically saves a copy of outgoing messages anyway) and not to the Sent folder in your “proper” account.