Issue Sending Email from a subdomain - Relay Access Denied

[MHC_1]

Server operating system version

Almoa Linux 9

Plesk version and microupdate number

18.0.74

We have just moved a set of accounts from WHM to Plesk. On WHM they all worked perfectly.

On Plesk, there is an issue that sending Script based emails (eg from PHP Script ; not from a mail client) from a subdomain to any 3rd party email account is failing because the subdomain is trying to send via the parent domain (a separate account on Plesk) . Both domains have working SPF records.

Maillog:

Dec 3 09:32:22 server postfix/smtpd[4138345]: connect from localhost.localdomain[::1]
Dec 3 09:32:23 server postfix/smtpd[4138345]: NOQUEUE: reject: RCPT from localhost.localdomain[::1]: 554 5.7.1 <[email protected]>: Relay access denied; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<admin.domain.co.uk>

SPF admin.domain.co.uk:

v=spf1 +a +mx +ip4:server_ip +a:domain.co.uk +include:domain.co.uk -all

SPF domain.co.uk:

v=spf1 +a +mx +ip4:server_ip +a:admin.domain.co.uk +include:admin.domain.co.uk -all

We have run

# plesk repair mail

Which has returned no errors or issues on any mail accounts.

What do we need to do to enable a subdomain to send emails outside the server and set as coming from the parent domain?

 

[MHC_1]

We have also moved the two domains on to the same subscription on Plesk but this doesn't appear to make a difference.

Here is also the core output from PHPMailer (only Debug level 2):

2025-12-03 12:06:29 SERVER -> CLIENT: 220 server.co.uk ESMTP Postfix
2025-12-03 12:06:29 CLIENT -> SERVER: EHLO admin.domain.co.uk
2025-12-03 12:06:29 SERVER -> CLIENT: 250-server.co.uk250-PIPELINING250-SIZE 25067520250-ETRN250-STARTTLS250-AUTH DIGEST-MD5 CRAM-MD5 PLAIN LOGIN250-XFORWARD NAME ADDR PROTO HELO SOURCE PORT IDENT250-ENHANCEDSTATUSCODES250-8BITMIME250-DSN250 CHUNKING
2025-12-03 12:06:29 CLIENT -> SERVER: MAIL FROM:<[email protected]>
2025-12-03 12:06:29 SERVER -> CLIENT: 250 2.1.0 Ok
2025-12-03 12:06:29 CLIENT -> SERVER: RCPT TO:<[email protected]>
2025-12-03 12:06:29 SERVER -> CLIENT: 554 5.7.1 <[email protected]>: Relay access denied
2025-12-03 12:06:29 SMTP ERROR: RCPT TO command failed: 554 5.7.1 <[email protected]>: Relay access denied
2025-12-03 12:06:29 CLIENT -> SERVER: RCPT TO:<[email protected]>
2025-12-03 12:06:29 SERVER -> CLIENT: 554 5.7.1 <[email protected]>: Relay access denied
2025-12-03 12:06:29 SMTP ERROR: RCPT TO command failed: 554 5.7.1 <[email protected]>: Relay access denied
2025-12-03 12:06:29 CLIENT -> SERVER: RSET
2025-12-03 12:06:29 SERVER -> CLIENT: 250 2.0.0 Ok
SMTP Error: The following recipients failed: --- : <-@-->: Relay access ---: <-@-->: Relay access denied
2025-12-03 12:06:29 CLIENT -> SERVER: RSET
2025-12-03 12:06:29 SERVER -> CLIENT: 250 2.0.0 Ok
2025-12-03 12:06:29 CLIENT -> SERVER: QUIT
2025-12-03 12:06:29 SERVER -> CLIENT: 221 2.0.0 Bye