Does anyone know the chances of an email going missing?

Does anyone know the chances of an email going missing?

It can occur.

Spamblocker. Active spamblocks can stop emails arriving if they match their spam criteria or happen to have extensions like .exe attached to them. (some gateways don't even allow compressed files)
Mail server Queue. Depending on how a mail server is configured might mean that the administrator during an update doesn't fix the Folder/File attributes for the right user/group. When this happens any mail sent to the queue can't be written, so it's as good as sending it to /dev/null.
Unauthorised access. If you are using an online mail service like hotmail, or POP3/IMAP then there is a chance that perhaps someone has gained access to your account and is accessing it from another location. This means that perhaps new emails might appear to have been read or not appear at all.
DoS Attacks (Denial of Service) or DNS errors. This for the most part doesn't occur very often however if an email is sent during a period when a mail servers DNS is down due to a DoS attack, the mail can't be sent and might silently discard the "unable to send" message.