Change in email configuration?

We use our private server as a smtp relay (with postfix and dovecot for authentication). And at some point in the last week or two it stopped working with dovecot complaining that the users it wanted to run as (dovecot & dovecotnull) didn’t exist. But I don’t think they ever did.

I got things up again by configuring dovecot with actual existing users but I haven’t been able to understand what changed - any ideas?

Dreamhost: is there a way to know when you are pushing our configuration changes to private servers?