vacation notice

I am using the trial version and have created an alias of an external email account (imap). I have tried setting up a vacation notice for the alias account. However, the automatic response is not sent when emails are received. Under 'advanced options' I have the choices to send automatic replies from by mailbox.org address and from 'default sender'. The default sender does not generate an automatic response and there is no option for my alias address.

My question is, is it possible to send automatic replies from an imap alias, and if so, how?

Many thanks