Last month, a staff member at work reported getting an email delivery status notification error when attempting to send to an AOL email address:
<XXXXXXXXXX@aol.com>: host core-aba03c.mail.aol.com[172.27.22.39] said: 554 5.7.1 null (in reply to end of DATA command)
The SMTP reply code indicated the receiving server rejected the sender’s email, but the “null” message didn’t provide any additional insight, and a search for 554 5.7.1 null turned up no solid leads. The staffer had corresponded previously with the intended recipient, so why, then, was her email rejected?
It turned out that the user attached an email thread—i.e., an Outlook item file—to her outbound message. (In Microsoft Outlook 2016, you can do this from the Message ribbon by choosing Attach Item > Outlook Item.) I asked her to save the email thread as a PDF file and send that instead, and when she did so, her message was delivered without any problems.
Interestingly enough, a user on the Gmail Help Forum later that same day—after our staff member successfully sent the email—reported getting similar errors when attempting to send email to AOL addresses. This may be noteworthy because we do use Gmail accounts via Google Apps. I suppose it’s possible that there was actually a problem with AOL’s server that was resolved between the time our staffer sent the email with the Outlook item file attached and the time she re-sent her email with a PDF attachment; even if that was the case, however, adding email attachments in unusual file formats is a good way to get your outgoing mail rejected, and it’s a good thing to check if you have to troubleshoot a bounced email.