Error when sending a letter from the server to gmail, where to dig?

Question

gmail

There is a server with mail on postfix + davot When you try to send a letter to gmail, the letter reaches spam and, in pursuit, the recipient receives an email with the error below. Where to dig?
This is the mail system at host fastemailservice.com.
I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The mail system
<[email protected]> (expanded from <->): bad address syntax
<[email protected]> (expanded from <-o>): bad address syntax
<[email protected]> (expanded from <16>): Host or domain name not found. name service
error for name=com type=A: Host found but no data record of requested type
(expanded from ): Host
or domain name not found. Name service error for name=com type=A: Host
found but no data record of requested type
(expanded from ): Host or domain name not found. Name service error
for name=com type=A: Host found but no data record of requested type
(expanded from ): Host or domain name not found. Name service
error for name=com type=A: Host found but no data record of requested type
(expanded from ): Host or domain name not found. name service
error for name=com type=A: Host found but no data record of requested type
(expanded from
): Host or domain name not found. Name service
error for name=com type=A: Host found but no data record of requested type
(expanded from
): Host or domain name not found. Name
service error for name=com type=A: Host found but no data record of
requested type
(expanded from ): Host or domain name not found. Name service
error for name=com type=A: Host found but no data record of requested type

Answer 1

To dig in the direction of whether the sender's address is correctly indicated in the message header, tk. gmail sees an address like [email protected], and such an address cannot be valid for obvious reasons.

Answer 2

How can I see the header of an email?
For FROM : [email protected]