Comment 14 for bug 164837

postinst scripts in tasksel are considered as maintainer scripts. So they cannot modify a configuration file. Patching dovecot.conf from the task postinstall script is forbidden.

So another solution would be:
 * modify the default dovecot.conf file to enable sasl by default in the dovecot package.
 * create a new package with a postinst script that would setup postfix with the postconf commands and depend on postfix
 * add the new package to the mail-server task so that dovecot and postfix are automatically integrated when the mail-server task is installed.