When you want to use your IMAP account as authentication for Postfix, you can set the SASL mechanism to “rimap”. However, by default, it will not supply the realm (domain) and therefore will authenticate with an incomplete username (john instead of john@bla.com).

To fix that, you need to add “-r” to the options in /etc/default/saslauthd (in Debian based distro’s):

OPTIONS="-r -c -m /var/run/saslauthd"