I found this (Change Helo/Ehlo address | Odoo):
I talked to hostgator about this. Apparently the Helo / Ehlo record of 127.0.0.1 is being blocked by their outgoing spam filters.
Checking the argument of the EHLO command seems to be an effective spam fighting measure when using SMTP. However, trying to do anything with this value when using authenticated SUBMIT (this is what a client like K-9 Mail is using to send messages via the user’s email provider) is quite pointless and bound to cause problems. Many mobile clients have no way of getting their (public) host name and/or IP address because they are behind NAT gateways. The SUBMIT server is in a much better position to do so (it sees the mobile device’s public IP address).
K-9 Mail 5.8xx is using the EHLO command with a “domain” value of 127.0.0.1. That seems to be the least bad option that works more often than it does not. I don’t want to make this value configurable because I consider SUBMIT servers that refuse to accept this value broken.
You seem to have the following options:
a) get your provider to fix their server
b) find another provider
c) use a different email client that works around your email provider’s broken rules