Why am I not able to receive email on mydomain.com when I am able to send emails?

If you are able to send email out, but unable to receive mail for a particular domain, there are several things to check.

The first would be to verify that your mail client (ie. Outlook, Outlook Express, Thunderbird, etc ...) is set to check the proper incoming mailserver (also referred to as the POP server). The next thing to check, is that the MX record for your domain is set to the proper value. An MX record is a DNS record that specifies which server incoming email should be delivered to. If this is set to an incorrect value, then mail might be being delivered to a different server than you expect.

Another reason for this behavior might be because the server you are using to send mail out from, might be incorrectly configured to accept mail for mydomain.com. When you send mail out through this server, it thinks that this mail is a local delivery and does not bother to check the MX record.