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@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?