getting error while running Notification Entry Dispatcher 1509 job queue for sending PO approvals

Hi, I am getting following error while running Notification Entry Dispatcher 1509 job queue for sending PO approvals

"the length of the string is XXXX, but it must be less than or equal to XXXX characters. A call to the mailkit.net.smtp.smtpclient.send failed with this message

SMTP mail setup can send test emails successfully.

Currently on this version of Business Central.

Version: GB Business Central 16.11 (Platform 16.0.22511.0 + Application 16.2.13779.0)

Any suggestions on this?