[CentOS] Sendmail and pmtu discovery

We have an issue with some customers who refuse to accept ICMP traffic
to their mail servers.  It seems that they have put Mordac, preventer
of information services in charge of their firewall policy
(http://en.wikipedia.org/wiki/List_of_minor_characters_in_Dilbert#Mordac).

My mail logs are showing that customers who specifically disallow ICMP
traffic have many "Connection Reset" entries in our logs:

Oct 14 08:00:50 mailsrv sendmail[2024]: m9ED0Yf5002021:
to=<customername at customer.org>, delay=00:00:16, xdelay=00:00:16,
mailer=esmtp, pri=42476, relay=mail.customer.org. [XX.XX.XX.XX],
dsn=4.0.0, stat=Deferred: Connection reset by mail.customer.org.

I have disabled pmtu discovery on our routers as well as on all our
outbound mail servers.  Is there anything else I can do on our side to
help the situation?