Reading between the lines, are you saying that you left Plusnet asking that your account be down-graded to email only? If yes, then the process is as follows...
When planning to leave, you MUST advise Plusnet that you wish to retain an email only account - not just leave it to the Gaining Provider Led standard service migration process
YOU should note all of the existing Plusnet Email addresses you have
When the migration is activated Plusnet FULLY closes the account for final billing purposes
Plusnet then resurrects the account as an email only service
YOU then have to recreate the original email addresses
Plusnet then restores your email archive
From 3 until 5 any email sent by anyone will not be routable (will have come nowhere near Plusnet) because the domain youraccount.plus.com or the 'target' mail addresses will not have existed. In short any mail attempted to be sent is not "recoverable". Your correspondents will need to try again.