There are a number of open source libraries that provide mail sending functionality. Using the libraries themselves should not be grounds for rejection from the App Store, but you should take care about how and what you send by email. Especially you should make it clear to the user that emails are sent, and that you do not send of anything compromising the users personal privacy.
The most popular email library seems to be
skpsmtpmessage. There is also a library called Pantomime but it has not been officially ported to iPhone, so denending on your needs you might be better off with the first choice.