That's fine and can be made to fit in with guidance given in earlier replies.
Since the Mensa service is forwarding only, instead of collecting the messages yourself (which has been assumed for earlier replies), you need to define (in the Mensa member centre - I assume) a real email address (a suitable Plusnet address) which the Mensa server will forward your messages to.
Sent messages go via relay.plus.net (as assumed for earlier replies) with your mail program set up to use your Mensa address as the "from" address.
Have you already set up a Plusnet email address? I'll assume you have and that you want to continue using that independently of your Mensa address. The most convenient way to handle this is to use
Manage My Mail to define an additional mailbox (eg mensa) with its own password. That will then serve as the delivery point for your Mensa mail.
You would enter mensa@username.plus.com as the forwarding address at Mensa. In Windows Live Mail you put your Mensa address in the "E-mail address" field, and mensa mailbox password in the "Password" field. On the second page you put username+mensa in the "Login ID" field. Replace "username" in this paragraph with your Plusnet account name.
Other entries on the set-up screens are as shown in the Plusnet Windows Live Mail set-up guide
here.
David
