virtual_alias_maps map lookup problem

If you're running 12.04 instead of 10.04, you'll need to install the dovecot-mysql package in addition to those listed in the library howto.

It could also be an authentication problem, or the database query could be failing for some reason. Providing the five lines before and after the line you pasted from the log would be helpful.

It's okay. I worked out that the mysql password was incorrect in one of the config files.