Funny bug, really happened: Put @ (full-width commercial; U+FF20) in a patron email address. And see that Letters.pm does not like it. The address will not be accepted, no mail sent. The address appeared to be copied from a page where the full-width character is used to discourage spammers. For the solution two directions immediately come up: Left: Change it where we edit it. Right: But suppose that we like the anti-spam effect of it (although this is actually minimal of course), we could do a replace in Letters.pm (and maybe some more places where we directly send an email). Your opinion is welcome.
Tested and confirmed. I would go to the left (accept the letter in validation JavaScript and while validation with JavaScript disabled, replace it before storing to the database).
This does not apply anymore to master. It is not possible in opac and staff to put this email address in the patron record. (This check was not in older versions of Koha.)