Ah; editing this posting on re-reading your question. I see you have an ISP? streamline.net? OK.
Some of the following won't apply to you.
Since this is the Mac OS X Server forum, are you running Mac OS X Server here? If so, then check the mail server log file as a start, and match up the open ports on the server with however you've got the client configured. Using the Server Admin tool, you have full control over both the server firewall, as well as what sort of access control settings are required of mail clients.
If you're running Mac OS X client and not Mac OS X Server (and as I might suspect), then your mail server settings are incorrect within your mail client. Check with your ISP or with whomever is providing you with mail services; look for a FAQ or equivalent information associated with the particular mail server. (Mail server configurations can and do vary.) Chances are, something within your client's SMTP server settings including the username, password, port and target host name are wrong.
Within the mail client, the sending of mail messages and the receiving of mail messages uses different paths, and different settings, and different IP ports, and can even (depending on the server) use different host names for sending and receiving mail.
Unfortunately, we can't particularly help you here. Not without a whole lot more information on your environment, and on your mail server, and on the current configuration within your mail client. Your ISP (if you're using an ISP here) tends to have detailed mail client configuration information available for their customers, and that's the best path to pursue this issue.