Vacation use different SMTP server

Hi,

in our setup, we have two SMTP servers:

• one for receiving messages (this one is using Dovecot's LMTP for delivery)
• one for sending messages (this one is used by our users as
  outgoing SMTP server)

Only the first one has list of local domains, local e-mail accoutns
etc. so the second one will ALWAYS use MX records for delivery - with
this setup, we can allow to our users to add any domain to their
accounts (for example, if someone will add gmail.com and create e-mail
accounts on it, it's complete safe because we will never deliver mail
for gmail.com locally). The only problem seems to be vacation messages
(maybe other messages send by sieve), as Dovecot is using the first
SMTP server for sending.

My questions: Can i set different SMTP server for sieve messages? How
exactly is Dovecot/Sieve choosing which SMTP server will be used?

Thanks.

azur