Great, another strange Apple bug I thought until someone was able to forward me the bounced email to my personal email account. Further insight in the error showed that the message was getting bounced back due to a blank subject line. As you can see from the example below its due to a blank or empty subject heading. Yes, apparently Apple has added this as an actual “Feature”. You can turn this off however by commenting out the only line of code in the file /etc/postfix/custom_header_checks.

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This is the mail system at host mail.xxxxxxx.org.
 
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
 
<user@domain.org>: host
mail.domain.org http://xx.xxx.xxx.xxx said: 550 5.7.1 empty subject
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Turn off the mail service before you do this. Once the mail service is off edit the file using sudo pico.

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/^subject: *$/ REJECT empty subject header in /etc/
 
into:
 
#/^subject: *$/ REJECT empty subject header in /etc/

Start mail back up again and you will notice that the blank subject line blues are gone away.