java - Android: how can I send verification email -

i'm developing android app, , after user registers, i'd send him verification email according mail entered in registration process.

i saw several answers here have same problem:

1. only gmail
2. send mail self , know username , pwd.

how can can send no matter mail provider?

i have php backend, better there?

i go back-end, several reasons:

1. if there central back-end handles administration, should in charge of user administration , confirmation.
2. sending email phone means more data usage user.
3. sending email phone works if user on network allows smtp-connections outside own subnet. many isps, example, not allow that.