Domain mapping and emails being returned

Hello,

I used your domain mapping plugin to map a clients domain to a multisite install, however, since doing so, the client hasn't received a single email. Whenever i send them an email it bounces back saying 'no such user'.

This is despite the fact i only updated to two A records on the domain and left the MX records as they were. Their email are hosted by 123-reg and that's how i intended for them to stay - the email account is still set up as it was but no emails are getting through.

Please help!