Frustrated...sent emails not being received

I am using k9 mail on Android tablet. I normally use Outlook on my pc at home. I am traveling so using k9 mail.
I have sent multiple emails to close associate to 2 different email addresses. He has not received them. He checked spam and we have talked about it. I also sent 1 email to a good friend and he did not receive it.
On my end everything looks normal. The sent emails are in my sent folder. I get no error message.
My email address is the service provider is bluehost.
I don’t know where to look…k9 or blue host. Not sure how to troubleshoot.
Thanks for any help

Bluehost appears to not like how K9 identifies itself when delivering mail and seems to toss it, rather than notify the sender of an issue or deliver the mail. See 5.8 showing as sent, but not always arriving at destination for a detailed discussion of this.

Thank you! I will stop using k9. Probably go to Outlook for Android.