EM Notifications fail for a specific user

We have setup EM to send email to two users after metrics are breached.

Both have database username and are both entered as administrators in EM setup. Both have e-mail address for their usernames and both are subscribed in notification rules.

Then we changed the metrics in order to receive an email notification. However only 1 user receives the notification.

Going into the Metrics notification details I see the following error.

Failed to send e-mail to user2@domain.com: Not connected (Rule Owner=SYSMAN, Rule Name=Database Availability and Critical States)

E-mail sent to user1@domain.com (Rule Owner=SYSMAN, Rule Name=Database Availability and Critical States)