STMP 421 Error Occurred - receiving message when trying to send an email

When accessing email via laptop, seem to get the following message quite frequently:
"STMP 421 Error Occured.  Unexpected failure, please try again later."
I wait a few seconds and the message goes away and I'm able to send without issue.  I have been made aware there messages haven't been received.  This has been happening for about the past month.  Any insight would be greatly appreciated!