Many websites refuse to accept email addresses of the form `firstname.lastname@example.org`, despite the fact that the `+sometext` is perfectly legitimate1 and is an advertised feature gmail offers for creating pseudo-single-use email addresses from a base email address.
My guess is that the developers of these sites think, because they’re either lazy or incompetent, that email addresses have more restrictions than they in fact have. It’s reasonable (and fairly easy) these days to check the syntax of the DNS part of an email address, because few people use non-DNS or non-SMTP transfer methods anymore, but the mailbox part is extremely flexible and hard to check accurately. A sane thing to do is just trust the user, and send a test mail to validate the address.
I picked on Yahoo in the title of this post: Yahoo are by no means the only offender, but I just signed up for a yahoo account, so they’re for me the most recent. Their signup form also refused to provide any guidance about why they were rejecting the form submission: I had to use my previous experience of sites wrongly rejecting valid email addresses to guess what the problem might be. Fail.
Footnote 1: According to my best reading of the relevant RFCs, anyway. See the definition of `dot-atom` in section 3.2.4 of [RFC 2822](http://www.faqs.org/rfcs/rfc2822.html), referenced in this context by section 3.4.1.