Spurious blank messages received in Mail on iPad2

[Samdottore]

For some time now, and also (I think) since recently applying the OSX 5.1.1 update, I have been receiving occasional blank emails - No subject, No sender, dated 1/1/1970 (i.e. zero date, I presume), "This message has no content" - sometimes several in a stream, which can only be removed by closing the Mail app and reopening it.
Can anyone tell me what's going on?
Is there any way I can prevent or cure his irritating behaviour?

Rgds
Sam