when your daughter hits send;
mail gets handed off from her computer to her Internet Service Provider (ISP).
the mail server set up by that ISP may filter or scan the mail, they may tag it, or they might just blindly pass it along.
It gets passed off to other servers & routers.. generally in the direction of your ISP.
there could be a direct connection from your ISP to daughter.. or there may be a dozen or more hops on the way.
At some point, a few seconds.. or a few days later.. the mail gets to your ISP.
Again, your ISP may scan, tag, or do other stuff to the email.. then they hold it until you try to download it.
At any point along the way.. from daughter's machine to yours.. the data that makes up your email could be lost, corrupted, or put 'on hold'.
If any of the servers along the way is down, even if just for a few seconds.. the mail might be put in a storage queue.
Normally, the convention is to try every few hours for about 3 days - to pass along files where there is a problem delivering it. BUT, convention isn't anybody's law. Nothing stops 'them' ( any them) from deciding to drop the email on one failed attempt, and no message back informing daughter of the problem. Or, we could hold your mail for months, and after a year send you a message about a failed delivery attempt (or deliver it that late).
will it be deleted? for most purposes, probably yes. nobody is going to really want to keep and store an email ; with all the information about who sent it and when.. for long, especially if they handle millions of messages a day. ( all those spam messages that get filtered out still count!)
-- can you count on it being deleted, say if you run for president in a year or ten? Nope. There's just no telling who may have copied it, saved it, and stored it.. whether the original gets to you or not.
will it be lost? again, for most purposes - yes. the mail will probably not be directed at the wrong mailbox; it will just sit on a server someplace until that server gives up trying to forward the mail.
Guaranteed? Nope. Nothing on the internet is guaranteed. anyone or many anyones.. could have copies of that email, and anything else. why 'they' would choose your email out of the hundred thousand emails that go past every second.. who knows? It is not likely, but it is not impossible.
*note: most often when I have a colleague or family member that says they sent the email; they usually are telling a little white lie... they forgot, or just plain didn't send it. Ask your daughter - if you really want to know - what the time stamp was on the sent mail, and if any logs on her end ( especially if work or at school).. show what happened to the email?