Something to keep in mind: RFC5321
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. 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.
Depending on how the filtering works, that could be an issue. If you're just checking that two emails from a one member are the same - that's OK. But if you're checking between users, I'm not sure as a case-insensitive comparison is a good idea.
RFC 5321 has several relevant items:
The local-part of a mailbox MUST BE treated as case sensitive.
Local-part = Dot-string / Quoted-string
; MAY be case-sensitive
However, exploiting the case sensitivity of mailbox local-parts impedes interoperability and is discouraged.
While the above definition for Local-part is relatively permissive, for maximum interoperability, a host that expects to receive mail SHOULD avoid defining mailboxes where the Local-part requires (or uses) the Quoted-string form or where the Local-part is case-sensitive.
So the RFC requires (MUST BE) that the local-part be case-sensitive, does not require (MAY) that it be case-sensitive, discourages depending on case-sensitivity, and says that you should avoid implementations where the local-part is case-sensitive -- all in the same document.
In practice, I don't know of any modern email providers that implement case-sensitive email addresses. The general rule in implementation is exactly the opposite -- most providers will accept any mix of upper- and lowercase characters. Essentially, case-insensitive email addresses have become the standard, even though the RFC specifies that they may be case-sensitive.
From an implementation standpoint, we have the choice of strictly enforcing the RFC and forcing mevans to fix whichever one of his email addresses is not "correct" (if he can even figure out why he's getting two copies of the message) or we can accept the de facto standard at the risk of a failure in the unlikely case that there are two Church members in the same scope whose email addresses differ only in their capitalization.
Restricting the case-insensitive check to a single member would reduce the already very small risk of failure in the sender's case, but still leaves open the larger case where members share an email address. According to the RFC, firstname.lastname@example.org
are different email addresses, but they both likely end up in the same place. If Bill uses the first version and Suzy uses the second version, a case-insensitive comparison for a single member will still result in them getting two copies of the message. I just did a quick check of my ward's records and there are three email address pairs like this where the same address (except for capitalization) is used for two different people.