I am being cautious to say it but updating to 10.11.5 removed my IMAP mail account and since reinstalling my details the PITA glitch where my outgoing server would not retain my password for more than an hour seems (hopefully) to have disappeared.
So far 10 hours with no problems (unheard of in the past since the last update).
Fingers crossed.
So far 10 hours with no problems (unheard of in the past since the last update).
Fingers crossed.