Yes, I monitored my kids e-mails. It wasn't because I didn't trust THEM, I wanted to make sure what they were receiving didn't have any weird stuff attached to it regardless of how innocent the kid sending it meant it.
In this instance, for one thing, it gives you the opportunity to talk to your daughter about chain mail. "IF you send this to 10 friends in 10 minutes....."
"If you DON'T send this to 10 people in 5 minutes...."
That stuff is for the birds and should not be taken seriously. At all.
Nor, in my opinion, should 99% of it be forwarded. Ever.
My friend let her son have an e-mail account. Fine. Well, they ordered something online from HER account, but when he received an e-mail about the "confirmation for his order" on his account, without thinking, he opened it and they got a bad virus that crashed their computer. They had anti-virus and security, etc. It could have been a worm, I don't remember exactly except that it messed things up.
So...even though you trust your daughter, I don't think it's a bad idea for her to just use your account and not open things without you looking before even opening.
And, you might let the other mom know what she sent and tell her that your daughter won't be opening forwards or mail with attachments, etc.
That's what I would do. Personally.
Just my opinion.