if you got to account settings. Select outgoing server (smtp) from the list. Select the SMTP server you are using and select edit. Change the password to something not encrypted. Check the SMTP settings against those your mail provider says you should use.

I've done that but there is no option for an edit

There are 2 SMTP setting locations in server settings. One to create and edit and another to assign them to accounts. Use the one marked create and edit in the attached image.