Emails not always being delivered but in sent folder

I have K9 on a Galaxy S23 and Tab S3
SMTP Server is setup on port 587 with STARTTLS.

If I send an email to an address in the same domain it is delivered and shows in the sent folder.

If I send and email to an address on Google or another server with the same hosting company (many many servers) the email is never delivered and shows in the sent folder. No errors or bounce emails.

If I install Outlook and use the same settings all emails get delivered.

Hosting company is going to say it is not their problem because Outlook and a Windows PC (Thunderbird) work just fine.

Sounds like the receiving MTA drops your mails based on the MUA Ident and/or IP.

Have you checked if your K-9 is set to use a local IP? Some users had problems with that in the past (depends on K-9 version; use forum search).

The search advice led to:

No updates since 1/2023.

Which is exactly what is going on. Strange that they let same intra-domain emails through but block inter-domain emails.

Wonder if Thunderbird is going to address this?
And when?

Thunderbird works correctly; it is K9 which has the issue. It’s an ideological issue which will likely never be resolved. The developer of K9 is in a war of ideas with major hosting services. He has taken a hard stand on user privacy, come up with an interesting method of protecting it which is standards-compliant, and major hosting companies are silently deleting the emails sent by K9 because they simply don’t like the way one part of the header looks. At one time there was an option to do it his more private way or the “normal” way, but he escalated his fight by removing the option. I believe that the developer is in the right, ideologically, but has picked a fight he cannot win, which will serve little purpose except to drive users away from his product.