After EE switch-over one device won't connect

[#1]

@pvmb  "Well yes, why wouldn't it?"

I don't know. I'm just prepared to clutch at straws to try and get this working.

In answer to your questions I have been able to log into my Greenby account. Interestingly the client settings for the email account don't seem to bear any relation to those in Thunderbird that have been working for months. This is what is shown:

and these are the settings for the incoming mail in Thunderbird:

I have also logged into webmail and tried sending a test message which appears to have been successful.

[#2]

@Buggwrit 

The normal settings for someone with a Plusnet account, using POP protocol would be -

Incoming Server Address:  imap.plus.net [91.204.208.26] (aka mail.plusnet.enmail.co)
Protocol: POP
Port:  995 (Encryption Enabled)

Outgoing Server Address:  relay.plus.net [212.159.8.107]
SMTP Port:  465 (Encryption Enabled)

However, you can see above that relay.plus.net is in the Plusnet domain IP address range. Whereas relay.force9.net [91.204.208.196] is in the Greenby/Enix domain IP address range, as is  mail.plusnet.enmail.co [91.204.208.26] (same IP as imap.plus.net/mail.plusnet.enmail.co) so, if you have now left Plusnet, you may have to use one of these servers instead of relay.plus.net  [212.159.8.107] to send mail.

Although mail.plus.net [91.204.208.26] that you quote as using for sending SMTP mail is also a Greenby/Enix domain host, it is the IP address for incoming mail - imap.plus.net [91.204.208.26]