I have noticed over time that Plusnet as not been one of the most reliable as far as email is concerned. I solved the problem a while back as follows.
My domains are hosted by Virtual Names
I set up a dialup account with Tiscali and dial in for a few minutes each month to keep it active.
On Virtual Names I have set it up so that all emails come to me via BOTH Tiscali and Plusnet.
If one goes down I still get the same email from the other.
To date I have not lost any.
The only downside is I now get every email twice and therefore every piece of junk email twice.
In all the time I have used this method for email Tiscali has only gone down about twice while plusnet, every few weeks or so.
As I have 2 phone lines at home (1 work, 1 home) am considering adding broadband to the other but will definately not use plusnet for the 2nd.