class-phpmailer.php - Duplicate header 'Date'

[jbrabo]

For servers using sendmail configured with Amazon SES service as instructed at:

​http://docs.aws.amazon.com/ses/latest/DeveloperGuide/sendmail.html

Sending emails via WordPress wp_mail will result in error: Duplicate header 'Date' resulting in mail not being delivered.

Error log:

---

554 5.0.0 Service unavailable
Content-Type: message/delivery-status
Action: failed
Remote-MTA: DNS; email-smtp.us-east-1.amazonaws.com
Diagnostic-Code: SMTP; 554 Transaction failed: Duplicate header 'Date'.

[bpetty]

I'm not seeing duplicate "Date" headers in emails in either 3.5 or 3.6. Can you please elaborate on what data you are passing to wp_mail() and also inspect which filters you have added for "wp_mail"?