Due to a customer having being stuck with a virus I found out this today:
http://www.secsup.org/complaints/
So I added these lines in our outgoing mail servers' sendmail.mc (for the future):
#Remember that sendmail distinguishes between LHS and RHS with TABS not spaces
LOCAL_CONFIG
F{MCI_DOMAINS} -o /etc/mail/mci-domains
LOCAL_RULESETS
SLocal_check_rcpt
R$* $: $>canonify $1
R$- < @ $={MCI_DOMAINS} . > $* $#error $@ 5.1.3 $: "Cannot send mail there. See http://www.secsup.org/complaints/"
with /etc/mail/mci-domains containing:
mcimail.com
internetmci.com
networkmci.com
internetmci.net
networkmci.net