When the CommuniGate Pro SMTP module connects to a mail host/relay and tries to establish a secure (SSL/TLS) connection, it receives the host Certificate and checks the name in that certificate. That name should match either the name of the domain the mail should go to, or the MX relay name for that domain name.

When a remote server hosts several domains on the same IP address, it always sends out only one certificate, because the server cannot learn to which domain the incoming messages will go to and thus it cannot present the Certificate for that particular domain. As a result, your (sending) server may refuse to proceed.

If the server mainhost.com also hosts client1.com and client2.com domains, and the MX records for all 3 domains point to the same name and to the same IP address on that server, the server will always present only one Certificate - usually, the mainhost.com Certificate.

To allow your CommuniGate Pro server to send mail securely to client1.com and client2.com domains, you should specify 2 Domain-level Router records:
client1.com = client1.com@mainhost.com.via
client2.com = client1.com@mainhost.com.via

These records will place mail to client1.com and client2.com domains into the mailhost.com SMTP queue. You should place the mainhost.com name into the Send Encrypted list of the SMTP module, and the server will connect to the mailhost.com server, check its certificate (it should contain either the mailhost.com name or the name of the relay the SMTP module connected to), and then the SMTP module will establish a secure (SSL/TLS) connection with that server and it will send mail to recipients in the client1.com and client2.com domains via that secure connection.