The one user in 10,000 who actually uses an ip address as an email address can surely figure out why they're getting rejected. Of course, he'll probably go channel the Comic Book Guy and write a scathing post on his blog (Worst. Regexp. Ever.), but the normal people who use the service probably won't see that anyway.
Again, what material difference does it make to you?
Overzealously rejecting valid addresses is an application of subjective and inaccurate ideas about what addresses 'should' look like, and ignores the simple fact we've already mutually and formally defined valid address formats via the IETF RFCs.
> Again, why does this matter to you, other than some sort of misplaced sense of authoritarian aesthetics?
Yes! Great Comic Book Guy impression.
Why it matters is that for most smallish companies, you want to get something up that helps your users not do stupid stuff (†), but due to time and resource constraints, you're likely to end up with some kind of 80/20 solution. It'll work well in most cases, and fall down in some others. I would certainly agree with the idea that you not force people, but a nudge is probably going to save you money in increased user retention and fewer support hassles.
† - I once had a person ask why their emails to http://example.com were failing.
In that case, great junior engineer impression on your part.
Pedantry matters in complex interoperable systems, because otherwise they're not interoperable. This is why we have detailed standards documents on e-mail address formats.
> I would certainly agree with the idea that you not force people, but a nudge is probably going to save you money in increased user retention and fewer support hassles.
A 'nudge' isn't going to come from yet-another-broken-email-validation regexp. There's no need for an 80/20 solution; this just isn't that hard.
> † - I once had a person ask why their emails to http://somesite.com were failing.
That's not a valid e-mail address (as per RFC822).
