Nagios email stopped working after changing the relay mailserver

My colleague mentioned the other day that our Nagios monitoring was very quiet for the last couple of days. I checked the maillog on the Nagios server, to see that all the mails were in status ‘Sent’. A closer look at one of the records showed that Sendmail was still using the old MX record for the target domain. This record had been changed about a week ago and a
[root@Nagios ~]# dig MX company.domain.com
came up with the new SMTP server. I thought that a restart of Sendmail would fix the problem, but it did not. Bummer!! 🙁
The only way to force Sendmail to use the new MX server was to add the following line into the /etc/mail/mailertable:
company.domain.com esmtp:[IP]
Where IP stands for the IP address of the mailserver in the MX record.