mail problems

i have installed my webserver with this tutorial: https://www.howtoforge.com/perfect-server-debian-wheezy-apache2-bind-dovecot-ispconfig-3-p5
all works fine except mail.

1. when i send from an external mailprovider (gmx or gmail)to an e-mail account on this server the e-mail dosent arrive and no errormessage comes back
2. when i login to squirrelmail i can write a mail to e-mail accounts on this server but not to an external e-mail (like gmx or googlemail)
3. in thunderbird i could login to the e-mail account(imap) and see messages on the account but i cant send e-mails (smtp server didnt respond). in ispconfig on the service tab i see smtp as online but when i check the ispconfig monitor tool on my phone i see smtp as offline!?

i use (Debian Wheezy/Sid) ISPConfig 3.0.5.4p5 on the server

any hints?

best regards joe