I stopped receiving email alerts from RTMT and found out that the customer updated their DNS servers' IPs and SMTP server's hostname. I updated DNS IPs in CUCM and restarted Tomcat then updated the "mail server" in RTMT to the new hostname but was still not receiving the emails. I verified CUCM could ping the FQDN of the relay server. I updated the mail server to use the IP, instead of hostname, and I am now able to receive the emails.
Could RTMT be looking to a cached entry for the DNS servers or something? Any thoughts on why RTMT logs show unknown SMTP host?
Logs from RTMT:
Apr 27 10:32:03 SGSNA-CM1 local7 2 : 1980: SGSNA-CM1: Apr 27 2020 02:32:03 PM.561 UTC : %UC_RTMT-2-AlertEmailNotifyFailure: %[FailureReason=javax.mail.SendFailedException: Sending failed;#012 nested exception is:#012#011class javax.mail.MessagingException: Unknown SMTP host: smtp.company.com;#012 nested exception is:#012#011java.net.UnknownHostException: smtp.company.com][AppID=Cisco AMC Service][ClusterID=][NodeID=COMPANY-CM1]: Alert Email Notification Failure.