Many sites reject valid email addresses. One character it is common for forms to reject is a "+" in the left hand side of an email address. The email RFCs allow this character, so denying it is bogus. Nevertheless, they do.

https://tools.ietf.org/html/rfc2822#section-3.4

atext = ALPHA / DIGIT / ; Any character except controls, "!" / "#" / ; SP, and specials. "$" / "%" / ; Used for atoms "&" / "'" / " * " / "+" / "-" / "/" / "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"

atom = [CFWS] 1 * atext [CFWS]

dot-atom = [CFWS] dot-atom-text [CFWS]

dot-atom-text = 1 * atext * ("." 1 * atext)

...

addr-spec = local-part "@" domain

local-part = dot-atom / quoted-string / obs-local-part