First you should test if the mail sytem of the local local server is ok:
1) Create a email domain in ispconfig.
2) Create a email mailbox in ispconfig.
3) Login to webmail (squirrelmail) on your server with the email address of this mailbox.
4) send a email from within the webmail client to the address that you used tologin to webmail and check a minute later if the email shows up in the webmail account. So if you lgged in with email@example.com
, then send in webmail a test email to firstname.lastname@example.org
if 3) or 4) did not work, then please post the error messages that got added in the mail.log file for these actions.