I not much well understand, as this do, but me it is necessary so:
prescribe in DNS
record "A":
my4.domen.ru 1.2.3.4
my5.domen.ru 1.2.3.5
my6.domen.ru 1.2.3.6
record "PTR":
1.2.3.4.in-addr.arpa my4.domen.ru
1.2.3.5.in-addr.arpa my5.domen.ru
1.2.3.6.in-addr.arpa my6.domen.ru
This for coming messages.
The Enterring messages come on one IP.
Probably, I shall not be able adjust the transport itself. Can I address to You that You helped me, as pay service?