Mac mail intermittently offline

My mac mail intermittently goes offline and am unable to send messages. This has been happening for the past couple of months, possibly starting after a software update. I'm currently running High Sierra 10.13.6 on an iMac Retina 5K, 27-inch, Late 2015.

The error message I'm getting is "The connection to host plff-pzqw.accessdomain.com on port 587 failed."

The Mail Connection Doctor error message says "Could not connect to this SMTP server. Check your network connection and that you entered the correct information in the account preferences. Also verify that the server accepts connections on port 587. If it does not, change the port number in the Server Settings tab of the Account preferences."

I have confirmed that I have correct info for sending/connecting to my SMTP server. I'm able to send email through webmail, so I believe the problem is not my email host. I have tried changing the port setting from 465 to 587 and selecting "Use TLS/SSL, none of these have solved the issue. I have also spoken to Optimum Online, my ISP who has been of little help.

The problem happens many times each day. Sometimes if I quit mac mail and restart it several times it will work for a while. Sometimes waiting 5-10 minutes it will send a the email in the outbox on its own.

It also is happening on my 2019 MacBook Pro 16". Others in my household also on Macs are not having this problem.

Not sure what else to try. Any help is greatly appreciated.

Thank you.