Workflow Notification mailer status: stopped with error

Workflow experts,

We just recently upgraded EBS 12.1.3 to release 12.2.7. Part of this upgrade project is to enable workflow for Oracle SCM implementation. This is the first time we are activating workflow for this instance. After successful setup of IMAP and SMTP, we attempted to start Workflow agent listeners and Workflow Notification Mailer service. Agent listeners started fine but the Workflow Notification Mailer service started for few seconds and then showed, "Stopped with Error". In the log file, FNDCPGSC64869.txt, error message:

AST]:1525702765605-1:pnupgrade122:10.1.17.202-1:1:20420:SYSADMIN(0):-1:Thread[outboundThreadGroup1,5,outboundThreadGroup]:10.1.17.202:65814:1525702763543:53:STATEMENT:[SVC-GSM-WFMLRSVC-64869-10006 : oracle.apps.fnd.wf.mailer.SMTPMessageHandler.sendMessages(String)]:Call back event {null}