Invsalid Server response when attempting to send email via xfinity smtp

For last several months have had trouble sending email through xfinity.

I use client eM Client

Incoming mail loads fine

Outbound mail is very spotty.

If I leave email client running, email is eventually sent, might take 4-8 hours before local outbox is cleared.

Have checked xfinity third party settings

Have confirmed client settings with xfinity instructions.

Successful immediate transmission is rare but does happen occasionally

Have seen numerous thread son this and have seen that Xfinity disavows any problems with SMTP servers.

today eM Client is reporting (removed my email address for security):

1:47:39 PM [Edited: "Personal Information"] [SMTP] MailClient.Accounts.ConnectionException: Invalid server reply when connecting: 'resomta-ch2-04v.sys.comcast.net resomta-ch2-04v.sys.comcast.net Too many sessions opened
           '.
 

Frustrating to know that eM Client settings are correct (email does get sent eventually) but I have to experience delays and leave my email client up and running to ensure outbound messages are sent.  Have had instances where it takes two days before client successfully connects to xfinity SMTP and sends message.

This began occurring late last year.  Up to that time, eM Client had performed admirably, Xfinity response time was quick, outbound messages sent within seconds.

Have had 2 eM CLient upgrades since this issue began occurring, no change to Xfinity performance with either (not that I would expect any)

I am confident this is an xfinity issue but have no idea how to troubleshoot further or how to escalate to achieve resolution.