Outgoing email randomlly fails with STMP error

I have two computers on the same netwrok, both with licensed emClient Pro installed. After several years of working fine, one system will almost alway fail at sending an email if it has anything other than simple text content. The error is:

[SMTP] An attempt to connect to [[email protected]](mailto: [email addresss]) fialed. This could be caused by temporary server unavailability or incorrect settings. Do you want to check the settings?

If the settings are checked, it reports Port: 587, Security Force usage of SSL/TLS.
I have done the diagnostic test of SMTP and it always reports success.

In looking at the SMTP log, at the failure it reports an exception such as decryption operation failed or unable to write to remote host. Connection was closed by remote host.

Sometimes a message will sit in the outbox for several hours and then finally go out. In the meantime that computer is able to use the internet etc. Once an outgoing message fails with this error no other messages can go out.

The other computer with same setup, on the same router (both hard-wired) never has this problem.

This creates a termendous problem because we cannot send useful information beyond a little bit of text.

Any suggestions?

If the SMTP settings in eM Client are exactly the same on both computers, then sometimes temporary server unavailability can be caused by optional programs delaying or interfering with the mail client or mail port etc.

So if you have any optional eg: Antivirus programs, Firewall / Security programs or VPN’s running on that computer (other than what’s built into the OS), try temporarily completely disabling all of those to test.

Failing that, if these computers are in the same room or close by, try moving the computer that is getting the error to the other LAN connection which does work and see if that still happens. You can then rule out if its anything to do with the physical LAN connection or if its a computer related issue.

I have disabled all antivirus firewall, VPNs etc. The problem persists. I have also tried switching computers. To no avail.

The logs indicate the connection was forcibly closed by the remote host. I am contacting the server for assistance in identifying the cause but have not been able to resolve it.

Thanks for your help. Any other ideas will be appreciated.