Missing check for mail-sending success

Some time ago our site got blocked outgoing e-mails (by the webhosting provider, due to spam issues), and so I discovered that Drupal's user and conatct modules are not checking for the return value of mail() function (via user_mail() alias drupal_mail() ) The resulting situation might be almost described as critical, because users were happily registering, but receiving no mails, then sending complaints via site-wide contact form, and no reply. All lost due to mail() failing, and watchdog-logs perfectly clean.

This was on 4.7.3, so I silently patched the modules, and assumed I can't say more, because we're not up-to-date with Drupal 5. But now, upgrading to 5.1, I see that the situation is exactly the same on 5.1. Note that in the requesting-new-password functionality, the code is OK.

Steps to reproduce:
--- Drupal 5.1 or 4.7.3 (and possibly others?), normal public user-registration with e-mail.
--- Configure your php or mail software, so that mail() returns FALSE (not sure how exactly, but I'm 100% sure that the return was FALSE in our case)
--- Register a new user.

Expected behaviour:
--- Error message to watchdog logs, and to the user.

What happened instead:
--- User got a green message, that e-mail was sent
--- Watchdog got no error/warning entry at all.

Solution proposal:
--- Take the success-check from request-new-password case, and include similar checks to all the places where e-mails are sent.
--- Alternative: Add error-logging to the drupal_mail() if possible.

Patch:
--- Attaching my patch currently used on live site, but this is badly incomplete, as there are other occurences of mail-sending, that we don't need (= can't test on live site).
--- Needs work.

(I'll submit an isuue for contact.module separately)