I think the most useful thing would be to find out where the "undeliverable" message is coming from. For that you need to look at the headers, which is a little like reading the entrails of a chicken...
How easy it is to see the headers depends on what email program you are using - on Eudora there's a little button marked "Blah, Blah, Blah" which shows/hides the headers, on Outlook (possibly not Outlook Express) you go to the "View" menu, then "Options" then you get a bit marked "Internet Headers".
The reason that these are important is that every server through which your message passes adds a new header to the top of the message with its IP address, a date stamp, and any error info. This will make it fairly clear where the message is getting sent back from as "undeliverable".
It's very weird for the bit before the "@" to cause an error in any mail system except the final recipient. If you send to
info@foo.com then any mail server handling this mail (your ISP's outgoing server, any number of intermediate ISP servers)
should ignore it unless it is the mail server at foo.com itself, which will parse this part to see where and to whom it should be delivered.
Whatever happens, you should have an error code with a bit more info (unless a spam filter is being a bit over zealous, but those usually just drop messages, not bounce them back). There should be an error code - the most likely being:
550 Requested action not taken: mailbox unavailable (E.g., mailbox not found, no access)
551 User not local; please try
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed (E.g., mailbox syntax incorrect)
554 Transaction failed
If you do find the headers and can't make sense of them, PM them to me and I'll try reading the runes for you.
