Few things to check; in no particular order:-
- do you have correct MX (mail exchanger) records pointing to the server for delivery of email? Something like http://www.mxtoolbox.com/
will help you determine whether or not your DNS is correct.
- You have the mailbox created correctly in the ispconfig interface? Would assume so as you suggest you can send email, but worth checking.
- what does the ispconfig monitor say? any problems with postfix? Is postfix running? what happens if you do a "service postfix restart"
- what does /var/log/mail.log say - any errors in there? Do a "tail -f /var/log/mail.log" on the console and send yourself some email from outside, see what happens in the log.
- Do a "postqueue -p" - is there anything queued awaiting delivery.
- Is amavisd running? any errors there that might prevent delivery of email?