I'm not an expert in this area (even though I'm a sysadmin type) there are several reasons why undeliverable mail may not be returned to you. I've listed a few reasons that immediately come to mind though I'm certain that there are many other plausible reasons as well.

Keep in mind how mail usually makes several jumps between sites before reaching its destination, and the fact that problems can occur at any one of those sites. Sometimes mail flow becomes clogged because an intermediate site is down and there's no alternate path to take. At one time, all incoming mail to my work site (Mobile, Alabama) had to pass through Huntsville before it could get here. If the Huntsville site was down, we couldn't send/receive mail until it recovered. Related to this scenario is one in which a "critical path" site has a problem with their Domain Name Server, which can sometimes become corrupted or--if an independent box--downed. Mail can be delayed for up to 3 or 4 days while waiting for the impaired site to recover. If the downed site doesn't recover within the alotted time, a return-to-sender attempt is made. So if it hasn't been at least 3 or 4 days since you sent the message, you may still get it.

Sometimes, a site administrator develops custom sendmail wrappers that try to filter/process mail for specific reasons. I develop custom filters that are used for bulkmail delivery purposes (note: our bulkmail is internal). Sometimes wrapper bugs can bypass normal sendmail processes and accidently trash incoming mail. The bug may be minor and escape the programmer's notice.

An unlikely but still plausible scenario is that although a recipient account has been login disabled, it hasn't been removed from the system. The mail is sucessfully delivered, but the intended receiver believes that the account doesn't exists anymore. I'm sure that I probably have at least 3-5 accounts of this type on my systems right now (makes note to self to check/remove locked accounts).

Another unlikely but possible situation occurs sometimes at my workplace. We have two major email servers: one Unix-based and the other Groupwise. It is possible for a departmental authority to call and request Groupwise accounts for a group of people. The people involved sometimes already have the Unix-based account and are unaware that they have a Groupwise account. Next thing I know users call wanting to know what happened to an important memo they know has been sent to them but they haven't received. The sender won't receive an undeliverable reply since the message was delivered.

Rather than focus on the "from" address, have you determined for certain that the "to" address exists and is deliverable? Sometimes resolving this type of problem can only be accomplished by contacting the postmaster of the delivery site involved.

--Jim