Error:

After sending the letter, it is returned with this error.

550 5.7.23 <sender@domain.tld>: Recipient address rejected: Message rejected due to:
domain owner discourages use of this host.
Please see http://www.openspf.net/Why?s=mfrom;id=mailto@remote-domain.tld;ip=xxx.xx.xxx.xx;r=<UNKNOWN>

The reason in this case is that the openspf.net verification service is unavailable and cannot verify the sender, and therefore does not return a positive response, which means letters sent to the address will be returned with an error.

Solution:

The remote server administrator needs to remove non-working hosts for SPF checks from the configuration file.

Using the example of Postfix mail server settings.