It depends a little on your email service and on what email software you're using. With "real email" programs, you have an Outbox, and a message "hung up" in the Outbox may repeatedly try to send itself. Again, in a "real email" program, once it's been sent (received by the "recipients" service, but not necessarily received by an individual recipient) the message is moved to a "Sent Items" box. Nothing in the "Sent Items" box should attempt to resend itself. It's a one-shot deal. Delete the file from your "Outbox" and the problem should disappear.

With most html programs that I've seen, there is no visible Outbox so you have no way of knowing that there isn't something in "your Outbox" on your providers server; but most systems work just as for "real email" where it's sent once, and immediately moved to a "Sent Items" folder.

An item may remain in the "Outbox" if your service was unable to "make a connection" with the service provider for the recipient. If you screwed up the address so that it's a "could be a real server but isn't" your service might try to resend later. There should be a "timeout" or "number of tries" limit that will eventually expire. If you don't have a visible and accessible Outbox, your only recourse is probably to wait for the expiration.

Since you're reasonably sure that the message you're getting is the result of a specific message you tried to send, you may not be the recipient of an attack; but you should know that "delivery failure" messages are a very prevalent spam/malware attack method. Since the notification of failure typically comes with the original message as an attachment, there is the tempatation to open the attachment. A malicious/fake message can contain virtually any malware the sender wishes in such a faked attachment.

John