Альтернативный порт sendmail

http://www.sendmail.org/m4/mailertables.html

For example, you may want to MX everything in a domain to one machine that then forwards it directly. To do this, you might use the DNS configuration: 
*.domain.	IN	MX	0	relay.machine
and on relay.machine use the mailertable: 
.domain		smtp:[gateway.domain]

Что тут не понятного? Или тебе не это надо?

Через создание своего SMTP MAILER_a
___________________________________________________________

Q3.39 -- How do I send using an alternate port?
Date: May 5, 2002
Updated: January 10, 2004

The port used for outgoing SMTP connections can be changed via the
respective *_MAILER_ARGS macros for the various SMTP mailers. In a 
default configuration, sendmail uses either the relay mailer (for e.g. 
SMART_HOST when no mailer is specified) or the esmtp mailer (when 
sending directly to the MX of the recipient domain).

So, if you want all outgoing SMTP connections to use port 2525, you 
can use this in your .mc file:

	define(`RELAY_MAILER_ARGS', `TCP $h 2525')
	define(`ESMTP_MAILER_ARGS', `TCP $h 2525')

If you want to use an alternate port only for specific destinations, 
change (e.g.) only the RELAY_MAILER_ARGS, and make sure the relay 
mailer is not used for anything else. E.g. you can have sendmail use 
port 2525 only when sending to your domain with this in your .mc file:

	FEATURE(`mailertable')
	define(`confRELAY_MAILER', `esmtp')
	define(`RELAY_MAILER_ARGS', `TCP $h 2525')

and then in your mailertable:

	yourdomain.com		relay:mail.yourdomain.com

This will force sendmail to use port 2525 for connections to 
yourdomain.com. Of course, change 2525 to whatever alternate port 
number you wish to use.

То что тебе надо, не делается в одну строку.

создай виртуальных юзеров (вирт. домен) и пропиши этот домен в mailertable, т.о. всех кто попадет в вирт.домен sendmail перешлет в лотус (для пересылки на порт 2525 либо настроить mailer либо iptables -j DNAT)