From a purely technical perspective (and by this I mean going down into nuts-and-bolts Internet protocols), e-mail addresses should always be lower case. This will come as news to those who have a corporate address of Firstname.Surname@company... but, coming from the opposite perspective, no-one would expect FirstName.Surname@ to be a different address to firstname.surname@ - and most computer platforms apart from Windows machines are case-sensitive in most of their operation.

However, the world has moved on since e-mail was invented and case should not cause the confusion it appears to have caused you. For the reasons above ba.com might be being a trifle over-zealous if some software developer somewhere decided that it should be important, but apart from an out-dated preference for The Rules I can't see a reason for it.

Ironically, BA keep sending me e-mails and using a completely upper-case address - but you didn't expect a computer system to be consistent, did you?