Perhaps you need a separate MX entry for each one of the peer servers, e.g.
mydomain.com IN MX 10 peerserver1.mydomain.ch.
mydomain.com IN MX 20 peerserver2.mydomain.ch.
mydomain.com IN MX 30 peerserver3.mydomain.ch.
mydomain.com IN MX 40 peerserver4.mydomain.ch.
If Ipswitch's knowledge base does not clear up the issue, send them a mail and ask. My questions have usually been answered within one day.
Imail 6 U/G, page 42, clear as a Suisse cowbell:
"DNS entries:
-------------
ipswitch.net
IN MX 10 mail1.ipswitch.net
IN MX 10 mail2.ipswitch.net
IN MX 10 mail3.ipswitch.net
mail1 IN A 1.1.1.1
mail2 IN A 2.2.2.2
mail3 IN A 3.3.3.3"
Len
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