In their defence...
There's not really any excuse for this, and I don't use their mail service so I'm not affected by it, but in all other respects PlusNet get more flak than they deserve. Go read their website (especially if you have a customer account) and tell me what other company in any business is so honest and frank about things, especially technical stuff...graphs of call waiting times, internal management reports openly posted (including ones with red flags on), a lot of technical detail about how their packet shaping works etc. I've had an account for 2 years (just using it for the connection) and it's been rock solid, fast at all times including peak, and all the customer communication I've ever had has always been open and straightforward. At least when they do mess up they admit to it and often post a lot of details about what went wrong and what they are going to do about it. I'd rather have that than megacorp PR bluster about unspecified "technical problems".
In PR terms they are an unmitigated disaster, and losing people's e-mail really isn't on, but I do get the impression that there are some serious, hardworking and knowledgeable techies at the heart. If BT stop running Plusnet as a separate unit and migrate everyone onto standard BT products I'm leaving straight away; I can't be bothered fighting my way through 10 layers of "support" asking me if I've turned my PC on or if I know what a phone socket looks like when I'm trying to report a technical problem.