You get the error message “Lync cannot verify that the server is trusted for your sign-in address”

When Lync Communicator discovers the Lync FE to log on to it uses the SRV Record If the associated server FQDN is resolved to a server that doesnt match the SIP DOMAIN then this error is presented.EG Below record is for DNS zone, Sip Domain is but target host is a .local FQDN

Add an A record ( for the FE Server) with matches the Sip and DNS, then edit the SRV record to point to this record.