Cannot send email - plusnet via greenby

[#1]

I am unable to send emails via 'relay.plus.net' for a plus.com email address.

The email was migrated successfully in December and has been working well, but last week we had to leave Plusnet due to a need to retain a landline number for a vulnerable user, and moved to BT (supported by Plusnet as a move to EE was not possible). It is uncertain whether this is relevant to today's problem but it should not be.

Today we discovered sending email was failing, although incoming mail can still be received.  The failure manifests as a login failure and prompt for a new password.

The email client settings (Thunderbird on a PC) appeared to be those required by Greenby.

Greenby support is by an AI Bot.  The first response was that the settings being used were wrong.  Although the mail portal states the settings require port 465 and encryption enabled, the bot insisted that we must use either port 25 or 587 with no encryption.  I questioned the bot but it was adamant that encryption had to be removed.

Changing those settings made no difference.  I have tried changing the user name in the SMTP server authentication on Thunderbird - the bot said it could be any of 'alias@user.plus.com', or 'user+alias', or just 'user' but trying all of these with the apporpriate passwords made no difference.

I have also tried troubleshooting using powershell and curl.  Apart from showing times when the login was being blocked due to too many failures, this has not enabled me to make any progress.

The bot eventually gave up and transferred me to an agent who will be with me shortly - over an hour waiting so far.

I have looked at the various messages on this forum and I think I have tried everything others have tried.  I am coming to the conclusion that this is simply a failure by Greenby related to the passthrough of the authentication via the plusnet relay to the Greenby server.  It seems that in several of the previous discussions the mail has 'just started working' and hopefully that might happen again.

Note inbound IMAP mail is working fine and webmail is also available, so I am certain all passwords are correct.

I am open to any other suggestions.  I know the right one is to migrate to a more reliable email system but I had hoped to keep the plusnet email address as this is for an old, vulnerable user who cannot cope well with change.

[#2 · Accepted solution]

SMTP relay.plus.net is still hosted by Plusnet, 
If your Plusnet a/c has closed you may have lost access to this SMTP server.

In place of relay.plus.net try : mail.plusnet.enmail.co    a Greenby SMTP server,
try with Port 25 & No encryption to see if it accepts your credentials.

If that works you can try going back to SSL/TLS via Port 465 or STARTTLS via 587,
but you may need to add a Certificate exception to your email program.

[#3]

Thank you for that a pity the Greenby bot seems unaware of the problem and solution.

I was able to get this to work with port 25 and 587/STARTTLS after allowing the certificate exception.

Thank you

[#4]

Good to have made progress.
We also found the Greenby AI Bot is not aware of the split hosting or the implications.

Currently Greenby email servers mail.plus.net & imap.plus.net and mail.plusnet.enmail.co all point to IP: 91.204.208.26

I suspect once the Plusnet email migration completes (in 6 months ???),  relay.plus.net will also point to a Greenby IP,  it may even become  91.204.208.26 at which point we will be able to revert to relay.plus.net and to use SMTPS or STARTTLS encryption without any more Certificate issues.