[Typo3] T 3.7.0 Mailform doesn't send, how to debug?

[Robert Zöhrer]

Hello,

I've a very strange problem with the standard mailform, which doesn't
send any mails. But testmails from Install-Tool arrived prompt in my inbox.

Form settings must be OK, becouse I tested it with the standard
settings, choosen by Typo:

# Example content:
Name: | *name = input,40 | Enter your name here
Email: | *email=input,40 |
Address: | address=textarea,40,5 |
Contact me: | tv=check | 1

|formtype_mail = submit | Send form!
|html_enabled=hidden | 1
|subject=hidden| This is the subject

Of course I added a valid recipient in the line below ;)

Because I had no idea whats wrong, I've upgraded to version 3.7.0 ->
with no success.

Next I tried the patch for class.t3lib_htmlmail.php, which removes
features for the 5th mail() parameter, becouse there are some troubles
with earlier postfix versions  -> with no success
Anywhay my provider told me, exim is used as MTA, so the 5th mail()
parameter seems NOT to be the problem.

The crazy thing: I've another site running version 3.7.0 at the same
provider (same cluster, same provider environment) with same global
T3-Settings .. and there the form sends without any problems!

There are only a few extension, the other site don't have:

tt_news, gsi_popup, w4x_backup

But IMHO these extension shouldn't bother the mailform.

Now I want to debug line by line, what's goinig wrong, but I've no idea,
how to do.

There is a

#debug(array($this->recipient,$this->subject,$this->message,$this->headers));

in class.t3lib_htmlmail.php, but when I delete the sharp '#' nothing
changes. Where should I see these debug infos?

In class.tslib_fe.php, where mailform-processing is startet (or not?),
there are lines like

$GLOBALS['TT']->setTSlogMessage('"Formmail" discovered ...

Where do I see these logs?

Do I have to st some global CONF, I don't know?

I've tried

$TYPO3_CONF_VARS['SYS']['enable_DLOG'] = 1;

but it also didn't help.

When I manually add a Debuf marker like

########################
echo "DEBUG MARKER";
exit;
########################

in class.t3lib_htmlmail.php or class.tslib_fe.php, I get no browser
output like this. Is the page-rendering & output before calling the
mail-functions?

If anybody has some ideas to debug this crazy problem I'd be very glad.

THX Robert