You have not really given enough info to fix this easily, but from what you have said, I think that your issues are DNS related. Mail to a domain (MX record) name which cannot be resolved will stay in the IMS queue until it times out, at which pointthe sender will get a NDR.
Firstly, pinging www.myco.com has nothing to
do with your mail to myco.com. These are 2 different DNS entry types. The one is an MX record, and the other an "NS" record. For someone to be able to send mail to "me@myco.com" there needs to be an MX record created on an Internet DNS server for "myco.com".
Secondly, if you mail server is retrieving internet mail from your ISP, you need to put internet DNS servers into its network configuration so that it can resolve external names. These would probably belong to your ISP, but can be any Internet DNS server.
Hope this helps you.
Duwane