My favourite are the ones where you're not allowed to use “the portion of your email address before the @” in your password. My email address is r@foon.uk. Fun times :D
I hate any requirements for passwords but why go for cornercase email?
If the PM specifies a requirement to filter out email addresses with a single character before the @ sign, then that's a different thing. Maybe there is a business requirement to filter out people who own a domain and give themselves a single character email address? In that case it should be explicitly written into the validation logic and not done as a side effect of sloppy coding.