sieve sending vacation message from vmail@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 vm...@ns1.domain.tld instead of 
u...@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?