Ahoj!
My strict mail server refuses “localhost” on the submission port. Can you send the host part of the sender as the EHLO host/domain?
If I’m sending mail from me@example,com, first send EHLO example,com (replace coma for dots). Other mail clients do that and it helps.
delta.chat sends EHLO localhost
There is an issue open already: HELO "localhost" rejected (smtp and submission port) · Issue #1847 · deltachat/deltachat-core-rust · GitHub
And a related post with a solution, maybe it helps to check if SMTP authentication succeeds:
Normally servers should not require FQDN for authenticated submissions.
I figured that much now and dropped the rule from my postfix config. Still, more joy if delta.chat would come forward more proactively. If I was not in the mood today, I’d stop there, which would be a pitty because now I find the app really well thought out and the design would suit even to fancy ladies… 
Thank you!
localhost is sent for privacy reasons. Some servers expose hostname in Recevied header (unless configured to remove it) to all recipients of the outgoing mail, so sending real hostname there is not acceptable. And even then, it is usually not an FQDN.
Sending some FQDN, e.g. delta.chat., could help, but even that exposes unnecessary info about mail client. What do other clients send there?
Yet another idea: Try to find out whether the hostname is an FQDN (like, if it contains dots, try to resolve it as a domain). If it is, send it (or maybe only try it during configure).
Problem: This is quite some work, not sure if it’s worth it.
As mentioned already, I’d rather keep the privacy profile low. Local IPs/host names reveal too much information. Not for the regular home user behind a NAT, but in campuses, corporate networks, aye. With a slight assistance from network admins, you could literally walk to the person who just sent the message and find them still sitting there. Not fun. I’ve seen the change has already been made for [127.0.0.1]. Thank you.