Receiving error 220 on SMTP transaction

[SMFEmail]

My thunderbird has just been updated to 91.2.1 (64 bit linux) and when I send e-mails I now get the following error:

"An error occurred while sending mail: Outgoing server (SMTP) error. The server responded: 220."

To my knowledge 220 is the response to the initial "HELO" in the SMTP protocol and from checking with Wireshark that is the transaction that is taking place.

Receiving e-mails is working ok.

I have tried rolling back the version to an earlier one (2 versions back from current) with no effect.
I do not know what version I was on previously and I am reluctant to go to far back in case I break my profile.

My partners Windows laptop also uses Thunderbird (a quite old 78.14.0) and works fine with the same mail server.

Any ideas on what I can do to find out the root cause of this problem and fix it ?

Thanks

[SMFEmail]

On further investigation the 220 is being sent on the initial connection from Thunderbird and is closing the connection after the error is acknowledged the initial HELO or EHLO is not being sent.

[tanstaafl]

If you have a second account with somebody else does it have the same problem?

What happens if you temporarily configure the problem account to use the other accounts smtp server? The main reason I'm asking is to determine if the problem is account specific or not.

What email provider are you using?

By the way, version 78.14.0 was released Sep 7, 2021. That seems fairly recent, just a different major generation.

[SMFEmail]

The problems is with all accounts this Linux machine, there is a mail server on the office firewall for which I currently have no details which buffers e-mails to Virgin Media, but as I said other Thunderbird clients on other machines (all Windows based) are working fine.
From the wireshark dump I see the initial connection request followed by the 220 from the server and then a disconnect from the client. I have the wireshark dump if that is of interest. I have worked through all the SMTP proxy and configuration options on thunderbird and they all act the same.

[SMFEmail]

Done a bit of poking on the mail server and found a slight miss configuration of the server host name. I thought that was the problem. However now according to Wireshark the 220 message is being replied to with EHLO from Thunderbird which gets a 502 not implimented message the connection is then closed. When I run the same test from a Windows Thunderbird the EHLO is followed by a HELO,MAIL,RCPT DATA and QUIT. This mail is transfered successfully. From what I have read the 502 is a valid response to EHLO and the HELO sequence should follow am I correct or is this old information ?
Thanks

[SMFEmail]

Sys admin guy found a later version of our e-mail server which supports ESMTP and the problem has now gone away. Thanks anyway.