Re: just to clarify.
Yes, I understood.
What I'm saying is:
- A mail app "submits" an outgoing message to the (an) SMTP server. Ends with the DATA command ... actual data ... ".<CR><LF>".
- At this point, the (an) SMTP server either accept that message, and if it does, then the message is its responsibility, not the mail apps.
- We know that you're not seeing any issues with the above part -- Apple's SMTP serve does *accept* messages from Aqua Mail.
Re: EHLO
K9 Mail's is (maybe) wrong, Aqua Mail's is right: RFC 2821 section 4.1.3.
If Aqua's was wrong, the SMTP server would have replied to the (malformed) EHLO with an error status code, which it doesn't.
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Two things I would try:
AquaMail settings -> composing and sending -> turn off "send user agent".
Per-account options (long press on the account) -> options and folders -> composing and sending -> turn off "send message-id".
( something somewhere may be set up to check the "validity" of Message-IDs, or to reject certain user agents... but as I wrote above, no trouble for me sending from @icloud to gmail and fastmail )