My dad says capitals used to matter (years ago) for email addresses but don't anymore. I'm fairly sure they never did because something like that involving DNS/MX changes would not change. Especially with no easy to find record online.
Let's look at this in pieces:
The domain part of the email address needs to conform to RFC 1034 and is thus (and has always been) case insensitive:
http://www.ietf.org/rfc/rfc1034.txt
The local part of the email address is handled by the receiving mail server and thus whether or not it is treated case-sensitively in theory depends on that server. Some mail servers choose to impose rules in addition to RFC 5322 (e.g. Gmail ignoring dots) or relax them (e.g. most mail servers in Asia allowing extended punctuation at beginnings and endings of addresses).
A mail server MAY in theory (while breaking RFC 5322) implement case sensitivity.
That said, I am not aware of a single one that does or has ever done so.