@Lionheartsails – Did you upgrade recently, if so do you remember when and from what version of K9?
If a message shows in your “sent mail” folder it has been accepted by your configured outgoing mail server (which you indicate is hostmonster). If that server subsequently rejects the message for some reason it should notify you.
From previous discussion:
https://forum.k9mail.app/t/5-8-showing-as-sent-but-not-always-arriving-at-destination/2481
it appears that hostmonster will accept mail from a client and then dump it, but never inform the authenticated user/sender of this action. The current version of K9 uses a setting on the EHLO that hostmonster seemingly doesn’t like. It should reject the mail at that point but instead accepts the message (hence it showing in your “sent mail”) and then dumps it later. While one might argue that K9’s EHLO is the problem, there is no guarantee that any message sent through hostmonster will subsequently be delivered (or rejected back to you).
The old version of K9 did something different on the EHLO so some poeple are reverting to that, or you can use a different mail client. In the end however you can never know whether a provider that takes the hostmonster approach has bothered to deliver your mail or not.