Unable to send mail, email receipt unaffected [SOLVED]

[darond]

I am unable to send email from my mailinabox (v61.1), likely occurring for some days. Error message accompanying the error:

Apr 26 22:13:18 box postfix/submission/smtpd[18577]: connect from unknown[mysourceaddress xx.xx.xx.xx]
Apr 26 22:13:18 box postfix/submission/smtpd[18577]: NOQUEUE: reject: RCPT from unknown[mysourceaddress xx.xx.xx.xx]: 550 5.1.1 <test-fxteytopn@srv1.mail-tester.com>: Recipient address rejected: srv1.mail-tester.com; from=<my@email.co.uk> to=<test-fxteytopn@srv1.mail-tester.com> proto=ESMTP helo=<smtpclient.apple>
Apr 26 22:13:18 box postfix/submission/smtpd[18577]: disconnect from unknown[mysourceaddress xx.xx.xx.xx] ehlo=2 starttls=1 auth=1 mail=1 rcpt=0/1 quit=1 commands=6/7

Issue:
NOQUEUE: reject: RCPT from.....550 5.1.1......Recipient address rejected: ....

exists for all users when sending no matter what external recipient. Issue does not exist when sending to an email address configured on the same box.

System Status Checks all green
apt upgrade/updates all done
sudo mailinabox run numerous times and box restarted

I have dug around in numerous system and error logs without anything obviously jumping out to me, other than errors/warnings which seem consistent with a normal install.

Any suggestions on possible routes of investigation? I’m at a loss to understand why it has stopped working.

[darond]

Identified two lines in postfix main.cf:

default_transport = error
relay_transport = error

Commenting both out:

#default_transport = error
#relay_transport = error

now allows email to be sent freely (and received).

Are these lines default entries in a virgin miab?

[darond]

@darond

Are these lines default entries in a virgin miab?

Having quickly spun up a virgin box I can confirm they are not.

I have also identified in this Stackoverflow post that having IPv6 enabled on a server can also cause a similar issue, although main.cf does not align to the below:

Change
inet_protocols= all
to
inet_protocols= ipv4