Moving house, moving email from BT

[#1]

I am moving house at the end of the month from a rural line which gives 55.5kB/s-261kB/s [my highest ever] to a town where Plus say I can expect 11mB/s and I will be charged about a quarter my present price as it is a big exchange. I'll also move my phone from BT to Plus and I am *hoping* it will be seamless.
I have to say the help I got with the move was the best support I have ever had and I just hope the engineers, BT I assume, who do the work are as good …
My partner will be moving in with me late, but while I have my own domain so can point it where I like they have two working addresses, one btopenworld and the other btinternet. I know one person who moved to Spain and kept a BT address for an age and the Plus support guy said lots of Plus users seem to have BT addresses. I know BT own Plus, but how does this work? Do the servers remain smtp.bt.com, etc? Does this last for ever? I can find no reference to this via Google or on the Plus pages.
Cheers, Colin

[#2]

It's up to BT Retail how long they allow the btxxx addresses to remain operational and BT Retail owning Plusnet should have no bearing on this.
The incoming mail server address (POP3 or IMAP) will continue unchanged. BT might not allow their outgoing mail server (SMTP) to be used since the connection will not be one of theirs. However the Plusnet server relay.plus.net can be used from a Plusnet connection, or using authentication with Plusnet account/mailbox credentials from a non-Plusnet connection. Plusnet don't place any restrictions on what "from" email identity is supplied.

[#3]

You can keep BTinternet or BTopenworld email addresses after migrating away from their service by upgrading to the Premium Mail service for £1.53 a month.

[#4]

Thanks guys, but can you clarify for me. My partner's BT addresses will never be deleted after she ceases her contract, and and one way or another she can actually access her mail.
I am a Mac user so I'd leave Apple Mail POP3 settings unchanged [she would be better off with IMAP so she could share mail with her iPad] but would it be possible to point the POP3 at a Gmail a/c and also use their SMTP server with the 'Reply to' field set to xxxx@btopenworld.com? My reason for asking is that I seem to remember that the Plusnet authentication is a bit of a pain when away from home [which an iPad usually is]. I would know how to do this in Apple Mail, but have no idea how to do it in Outlook, though of course I assume it is possible.
Cheers,
Colin