Is mail.btinternet.com the appropriate SMPT server for the affected account? Does it happen for incoming messages sent to any recipient address (you), or only some of them?

If for instance you are trying to forward messages sent to an older email address (but forwarded to your current one by your ISP), I would think Thunderbird wants to send it using the SMTP settings as configured for another identity in Thunderbird, so check the SMTP server settings and Identity settings. Each identity can have its own SMTP server.

See Using Identities (bottom section about forwarding) for more info.

You may be able to see a more detailed error message when opening the Activity Manager, or in the Filter log for that filter.

Does this help?

Hi Tonnes and thanks for your reply. mail.btinternet.com is the correct SMPT server. I am able to manually send and receive emails, but when attempted via the message filter function I receive the error message. The message filter seems to be functioning as the filter log notes the message/s as forwarded.

bt.com uses yahoo for email. This may be the ongoing Yahoo/SMTP/SASL problem.

Or maybe not, as the OP is able to send/receive when using it directly. Hmm.

I understand the SMTP server is correct but again, does it happen for any recipient address of yours (To: in the filter drop-down) in case you have more?

For instance, if you previously had a btinternet.com address, it’s btmail.com now, your account and possible identities (?) are set up for the latter, and forwarding fails just for the btmail.com address, something may go wrong there. Also, is your account set up as POP or IMAP?

Replies to these questions would be welcome for bug hunting or reporting, especially since the filter reports the message was forwarded while it was not.

Aditionally, does the same thing happen when starting Thunderbird in Safe Mode? If so, an add-on could affect proper behavior.

Starting in Safe Mode makes no difference. The account is setup as POP. I've just tried the auto forwarding with an IMAP gmail.com email identity in Thunderbird and it works fine. Back in the email identity that I'm trying to get to work...I've created a message filter to forward the Mozilla support emails I have received from you to my other email address and they went first time no problem. However, when the message filter is used to try to send eight emails that I have received from sharepointonline.com, one of the emails makes it to the destination email and the other seven don't make it. I receive the time out error message seven times....so could it be something to do with where the email has been received from??

The emails that won't send are alerts from no-reply@sharepoint.com and just contain a link to an online file.

Thanks for the info, though I still don’t know about the recipient address (the To: field in received messages, so your address) for the incoming mails for what is probably your POP account. :)

I was trying to figure out whether the filter is "susceptive" to different To: addresses. In other words, the question is whether forwarding both succeeds and fails for different To addresses, or only fails for an older one that may be delivered to Thunderbird due to forwarding by the ISP but wasn’t recognized by TB because it’s not set up there. If so, they could also (but not necessarily) relate to possible identities that you may have set up in addition to the default one, as the Filter settings are based on accounts, but not identities. (There have been issues with identities in the past as far as I’m aware, hence the thought.) But if this is all happening randomly with one address and there are no other identities configured either, it does not apply. Besides, TB thinks or at least reports it was sent while it wasn’t, so that would be a small bug at least. Do alledgedly forwarded messages however show up in the Sent folder, I now wonder? All this is based on the text in the KB article:

When you reply to a message or forward a message, Thunderbird scans the recipients of the message and, if available, chooses the identity the original author used when sending the message to you.

Speaking of, identities are not accounts. They include header properties only and can be added to accounts in order to add other names, email addresses or more and allow you to send using different from: addresses for instance, if the ISP allows it, which is where things could go wrong in case of "unallowed" identities.)

Looking at the message that made it and the others that did not, it seems the issue is POP related at least, and not depends on the original sender, since the first one succeeded (assuming it was the first, which is hard to tell.) I could imagine the SMTP server does not allow sending more than one message in a row or severeal too quickly so to speak (and regardless of sender or recipient), or in other words, perhaps implemented some bulk mail prevention feature.

Therefor I looked for a preference to delay the auto forward process a bit within Thunderbird but haven’t found a way to do so other than perhaps by trying some add-ons. You could have a try with some listed on this page, though I’m not sure if they work for auto forwarding based on filters. In particular the Quote and Compose Manager got my attention (see the comment on its page, but again, I think it will work for newly composed messages only. Note that the add-on’s page itself will give a certificate warning - you’ll need to enter http in the URL bar instead, or just click here. Do note that many add-ons created by its author are generally outdated, and this one is supported up to TB 45 officially.

Nevertheless, this would be a (temporary) workaround unless something has changed on the server if it worked previously, or your ISP could provide some info about what the maximum allowed "send rate" is. If you can set up another POP account somewhere else (such as by adding Gmail as a POP account if you don’t have any) and set a filter up in the same way, you could pinpoint that, or perhaps compose 5 similar messages manually for the affected POP account and send them quickly in a row to see what happens then. On the other hand, you could also try to set up the current POP account as an IMAP account, as I think IMAP offers a bit more flexibility with regard to SMTP and IMAP traffic at the same time.

If you are really interested in debugging and viewing log files, you could even start Thunderbird with logging enabled to see what the SMTP server says during the forward attempts.

Did it start occurring at some point, or perhaps after a Thunderbird update? If the issue started recently, you could safely uninstall and reinstall an older Thunderbird version. If not, better not do so.

This will keep my attention though I won ’t be able to reply within a number of hours - perhaps others can jump in if needed. Please keep us posted.

Thanks for your help Tonnes...I found a workaround using Add-on 'RedirectFilter'. In TB's message filter menu it gives the option to 'Redirect to' instead of Forward messages. Additional settings enable a copy to be saved in Sent folder and the message to be marked as forwarded. It worked first time with no changes to any of the outgoing server settings.