Go into the web mail facility of your e-mail provider after she sends the mail and without e-mail program active. Does it appear there?
Then start on your e-mail program. If it's pop3 and set to delete it from the server when downloading it must disappear from the web mail and appear in your local inbox. Of course, it's different if you use imap or have the account settings to not delete received mail from the server.
This experiment helps localizing where the trouble starts.
I have experienced this (as a sender) when sending mail to some hotmail or compuserve account, when my email provider (not my account, but the whole provider, so it affects all their customers) is marked as a suspected spammer by some service used by your email provider. I complained with my providers customer service and they took a week to correct it. If that's the case with you also, it's something for your daughters provider to correct.
Until it's solved, all you can do is open another e-mail account (a gmail-account for example) or tell your daughter to communicate with you via whatsapp on your smartphone in stead of sending mail.
I have been exchanging emails with my daughter for a long time now. We are both using MS Office with Windows 7.
Quite suddenly a few days ago her messages began failing to reach me.
We've checked junk mail settings and her outbox but there's no obvious reason why she can't communicate with me. She can receive messages from me and her own email seems to be working normally with other people. My email also seems to be working normally with other senders. What is also disturbing is the fact that messages that are failing to reach me are not bouncing back to her with an error report, so where are they going and why are they not coming to me? She has tried deleting me from her contacts list and re-entering my address but this has not solved the problem. Any suggestions?