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.
This is a subjective question, hinging on the definition of "email."
My opinion is that your dad is right. Email does not only mean SMTP, RFC compliant email, especially in a historical context. cc:Mail was a corporate messaging system. AOL had internal email before they had an internet gateway. Etc, etc. Some of these older systems were case sensitive. I recall that when sending to some corporate gateways we needed to make sure the address was all caps for successful delivery.
Email started to become synonymous with SMTP in the mid 1990's.
http://en.wikipedia.org/wiki/Email#Email_networks
or
No.
By convention, e-mail names are treated as case-insensitive.
However, per RFC 2181:
http://www.ietf.org/rfc/rfc2821.txt
The local-part of a mailbox MUST BE treated as case sensitive. Therefore, SMTP implementations MUST take care to preserve the case of mailbox local-parts. Mailbox domains are not case sensitive. In particular, for some hosts the user "smith" is different from the user "Smith". However, exploiting the case sensitivity of mailbox local-parts impedes interoperability and is discouraged.
Another document said the local part MUST BE treated as case sensitive, see page 13 or search "sensitive" then you'll see:
Numerous different address formats used to be common on the internet, and with some of them, the text before the hostname was case sensitive. See obsolete addressing http://tools.ietf.org/html/rfc5322#section-4.4
In theory: yes
In practice: no
It is possible to make the part of the email before the @ case-sensitive, but it's never done.
来源：https://stackoverflow.com/questions/9013726/did-capitals-ever-matter-in-email-addresses