The announcement by Plusnet that their email processing would be moved to the Greenby company was because BT (who own Plusnet) decided that Plusnet will just be their 'budget' broadband service and little else.
This is a massive excercise, started some months ago and it not yet complete. If you have had a "Welcome to Greenby" email, then you must have been in one of the groups of users who have been migrated. Note that this only affects your email: your broadband internet access is still provided by Plusnet.
Although we were all told that users would have to make no changes, there have been many instances where for one reason or another changes have unfortunately been required.
Regarding your message:
"one of the recipients was rejected (or not recognised) by the server, Account 'mail.plus.net', SMTP server 'relay.plus.net' ".
Are you sure that is the exact and complete error information? It looks strange to me, as 'mail.plus.net' is not an account, it is the server name for incoming emails, usually the POP3 server.
Do you get the same message whatever email address you send to?
Are you able to find the server settings for the outgoing (sending) SMTP server?
They should include SMTP Server name, Port number, Security settings, username (but do not post your actual username)
Have you tried signing on to the Greenby website and then using View Webmail to send an email?
