Some mailboxes (SMTP servers) are picky about sudden surges of activity from a particular address, and may block the incoming mail from that address. If the mail gets received and thrown away at the mailbox there's nothing to be done about it, but it's not uncommon that the mailbox rejects incoming mail or bounces it back.
What happens in this circumstances? Does this software detect rejected and/or bounced mails? Does it notify the user when he/she logs in?