LinuxQuestions.org - SendMail Headache ! (& sleepless nights as well ;-))

Hi There,

i hope i ask a descent question here. i spent the last 5 days almost full time (and beyond) searching forums, doc's faq's the lot . but i can't get it right

i got a clean slackware 9.1 install with of course sendmail and bind defined at install

now, i have myhost.provider.nl AND mydomain.net coming out at my internet ip. now at my private network i decided to name the new slackware machine as following
sandcastle as hostname and mydomain.net as domain name (during installation (that would be the netconfig part)).
now when i send a test mail the following happens:
it is coming from root@sandcastle.mydomain.net
the other side smtp server can (of course) not resolve this domain name and therefor bounces the mail.

the most bizzare thing is that by coincidence i sent a test message from a clean redhat install, wich is by the way totally named incorrectly , namely, myredhat.provider.nl wich is totally wrong, but that message arrives properly ! if i look in the header i can see root@myredhat.provider.nl (myhost.provider.nl) <- CORRECT!

this is really driving me nuts, and totally lost in bind / dns documentation wich is a whole other quest (for next weekend probably :-) )

today i tried the following:
i edited my slackware box's resolv.conf with nameserver myredhat
now it sends out most mail, but the delivering mailservers that explicitly verify the sending domain (or perhaps run old software?) still bounce the mail
i can send to hotmail, but not to my company for example. the bouncing message states the same problem as above, could not resolve myhost.mydomain.net

ARGH!
i already tried MASQUERADE_ALL WITH all envelope statements using mc and m4 etc. but in the end i always seem to replace all with the original files !!

Hoping to have posted a descent question..

i'll have my breakdown now :-)