Change the outgoing method for an IMAP+ account
There is no user interface to change the outgoing method of an IMAP account.
If you used Sendmail so far but now want to use an SMTP connection instead:
Close Evolution.
Find and open the corresponding account file in $HOME/.config/evolution/sources/ with a text editor.
In the [Mail Transport] section, replace BackendName=sendmail with BackendName=smtp or vice versa.
Restart the evolution-source-registry background process, for example by rebooting your system.
Start Evolution and edit the account SMTP settings for the account as needed.