sieve sending vacation message from vmail at ns1.domain.tld

[Matthew Broadhead]

I have a server running centos-release-7-2.1511.el7.centos.2.10.x86_64 
with dovecot version 2.2.10.  I am also using roundcube for webmail.  
when a vacation filter (reply with message) is created in roundcube it 
adds a rule to managesieve.sieve in the user's mailbox.  everything 
works fine except the reply comes from vmail at ns1.domain.tld instead of 
user at domain.tld.  ns1.domain.tld is the fully qualified name of the server.

it used to work fine on my old CentOS 6 server so I am not sure what has 
changed.  Can anyone point me in the direction of where I can configure 
this behaviour?