Let vacation plugin ignore envelope sender to avoid SRS recipient

This is done by setting sieve_vacation_to_header_ignore_envelope to yes
The envelope is rewritten by recipent_canonical_maps to reverse SRS after the plugin checks it
so we need the plugin to ignore it at this point.

core/dovecot/conf/dovecot.conf

@@ -173,6 +173,9 @@ plugin {
   # Include the recipient in vacation replies so that DKIM applies
   sieve_vacation_send_from_recipient = yes
 
+  # Use To: header from original message becaus envelope has a SRS address
+  sieve_vacation_to_header_ignore_envelope = yes
+
   # extract spam score from headers
   sieve_spamtest_status_type = strlen
   sieve_spamtest_status_header = X-Spam-Level