I first confirmed that another mail client would send to my gmail account correctly, and it did. Same with Thunderbird on Windows. I can send from K9 mail to my POP3 business account, and receive it. When I send from K9 to gmail using my business account and POP3, the same setup that works to another business account in the same domain, the email goes into Sent but it’s never received by Gmail. I’ve done all this four times with the same result. Sending to gmail on the same business account, using Thunderbird on Windows, works. What’s happening?
If the message can be found in the Sent folder, K-9 Mail has successfully delivered the email to your provider’s outgoing mail server.
Check the recipient’s Spam folder. If the message isn’t there, contact your email provider to find out why the email wasn’t delivered to the recipient.