Hoping someone can help me out here.

I have a server sending our reports with bash scripts. We changed mail server and now the emails are being rejected with the help command by the email server below (I have changed the domain and ip in the log)

if I run hostname it displays as server1
if I run hostname -f server1.mydomain.co.uk

In
/etc/postfix/main.cf
I have tried adding the line
Yes email still rejected with help saying it's server1.localdomain

And advise would be great thanks

Do you have myhostname set in main.cf? https://www.postfix.org/BASIC_CONFIGURATION_README.html
Also, I am sure you know this but just want to confirm, after making changes you restarted postfix right?

Thanks for coming back to me. Yes I have also tried setting myhostname and after did a postfix reload and also tried a restart of postfix just incase

You should use masquerading

You need to change your HELO/EHLO from server1.localdomain to an existing DNS name eg. yourserver.yourdomain.co.uk. Make sure the DNS entry is there and is pointing to the public IP of your server. Also make sure the reverse DNS entry for your IP points to the same name to avoid other delivery problems.

Check your "myorigin" and "myhostname" setting in /etc/postfix/main.cf and/or content of /etc/mailname file. Your "server1.localdomain" value should be somewhere in you config. If you don't find it in main.cf try grepping your /etc for it.

Have you checked in /etc/hostname?

Is your DNS resolving the FQDN correctly?

Is your MX record pointing to the FQDN?

What is postconf myhostname?

Is your PTR record correct?

What is Postconf mydomain

Does postconf my origin return $myhostname?

Have you done sudo postconf -e "mydestination = yourdomain.com, \$myhostname, localhost.\$mydomain, localhost"?

@Ladowny's suggestion to grep for server1.localdomain may give you the answer.

What's in /var/logs/maillog? If you could sanitize entries where the error is shown, it might help.

Try installing mailx and sending testmails and then check the /var/log/maillog and post the sanitized results.