Email not being received

[#1]

My client has 46 email addresses set up using the default destination for their account. At the weekend sending to them stopped working with 550 Invalid Recipient error messages. No settings were changed at the PlusNet account I am told. However the client is using a destination domain which is external to PlusNet. It is a domain hosted by 1and1.co.uk. The MX record for that domain is set to
mx.avasin.plus.net
Tech support at PlusNet suggest the problem lies at 1and1 who assure me it doesn't.
The destination addresses still work to send and receive and oddly, they arrive with the 1and1 domain as the destination domain. Can you please suggest a fix before I create 46 new email accounts?

[#2]

Quote

. It is a domain hosted by 1and1.co.uk. The MX record for that domain is set to mx.avasin.plus.net

Has this always been the setup for this domain? I ask as I'm not sure how our domain system would associate that MX record to a specific account and therefore point the mail to the right place, which would explain the 550 error.

[#3]

Yes. There are no email accounts set up at 1and1. They are all at PlusNet and all worked until Monday. I see your point though.
Is it possible that by simply invoking that 1and1 domain by using it in the address my client was able to connect their account with it through some open channel in the PlusNet system which you have recently closed?

[#4]

Unless the domain is hosted by Plusnet I wouldn't have expected this to work (ever) for the reason stated by Chris. To make it work mail would have to be forwarded by an external Mail Transfer Agent to the default (username-based) destination domain for the account.
David

[#5]

But it worked for years.
Moot point now of course.