[ALUG] Delay in sending mail

Hello, Can anyone suggest why sending IMAP mail from either icedove, thunderbird, claws on a client is taking about a minute (about ten seconds "connecting to server" then over 30 seconds with "delivering mail"), whereas using mutt it goes in an instant? I must have something misconfigured somewhere, but don't know where to start looking - exim4 and dovecot logs on the server aren't showing anything up of interest. Thanks for any suggestions, Jenny