Receiving email via an MX Record on a non-Plusnet web hosting server

[#1]

Can anyone explain some details of how email 'forwarding' to Plusnet, using an MX Record on a remote server might work? I can't quite understand how it might work in detail!
My ISP is Plusnet and I would now like to use another company for web hosting.
I currently have 2 domains hosted on Plusnet, so that emails addressed to a@domain1.com and a@domain2.com both get delivered to a@username.plus.net.
If I move 1 of these domains to be hosted on the web-hosting company's server, I understand that I need to delete the domain entry from Plusnet account and add it on the web hosting account, so that web requests get resolved by the web hosting server.
As I understand it, it should then be possible to add an MX Record to the web hosting account that will forward email sent to a@domain1.com to my Plusnet account, so that email will not be affected - i.e. I will still be able to access email to all my domains from the Plusnet mail server.
If that is possible, what hostname do I need to put in the MX Record on the web hosting server? Is it "username.plus.net", "mail.username.plus.net", or the normal Plusnet SMTP server (that doesn't sound right)?
This is where my knowledge gets even more hazy: What then happens when the Plusnet mail server receives emails addressed to a@domain1.com from the web hosting mail server? Would it just ignore the 'domain1.com' part and just pass it to a@username.plus.net, or something more complicated, because "domain1.com" is actually hosted on the web hosting server?
Confused...so thanks in advance!

[#2]

I think the simple answer to that is that it won't work. Once you've transferred your domain away from Plusnet only your new hosting company will know anything about the domain and the MX record will have to point to either your hosting company's or your own email server.
Assuming you are not using your own server your mail will be received by your hosting company. Using whatever facilities they provide you can forward (or redirect) a@domain1.com messages to a different address which might be one of your addresses at Plusnet (a@username.plus.com or a@domain2.com), or anywhere else you choose, but you can't forward using an MX record.
Taking a look at a redirect in Manage My Mail and the help that goes with that might help explain the mechanism.

[#3]

Quote from: spraxyt

I think the simple answer to that is that it won't work. Once you've transferred your domain away from Plusnet only your new hosting company will know anything about the domain and the MX record will have to point to either your hosting company's or your own email server.

Not completely correct as the Web Hosting company may not be hosting the DNS. If you keep PN as the registrar and therefore keep the nameservers and DNS records at PN and only request the other company to host your website then you only need to change the DNS records for your website. All MX records will stay the same and hence your current email setup is not affected.

[#4]

Quote from: itsme

Not completely correct as the Web Hosting company may not be hosting the DNS. If you keep PN as the registrar and therefore keep the nameservers and DNS records at PN and only request the other company to host your website then you only need to change the DNS records for your website. All MX records will stay the same and hence your current email setup is not affected.

However that might not be possible either. Depending on the hosting providers set-up they may insist that the authoritative name servers are switched to there's. If this needs to happen then there's no way to create MX records to point back to our platform (you could however our servers would refuse the messages). You might have to rely on an email forwarding service as alluded to by spraxyt.

[#5]

Quote from: Bob

Depending on the hosting providers set-up they may insist that the authoritative name servers are switched to there's.

Are you thinking of PN as an example of the above statement? If so it's not completely true as when you request PN to host only you have to change the nameservers to PN but after it has been setup you can move the DNS to another provider and enter the A and MX records back to PN.