Stricter email format verification is needed before we send the mail.

[isoos]

e have some error log entries on the form "Response from server:

< 553 5.1.3 The recipient address <[abc]@example.com,[cde]@example.com,[efg]@exampel.com> is not a valid RFC 5321 address.

[isoos]

I think we don't want to do more than #8404 (not allowing comma in email addresses) at this time.