What are email duplicates and why are they bad?
Email duplicates contain content identical to another message sent from your account, most importantly unique identifiers called headers. Duplicates often trigger our spam filters, preventing the original email, along with any duplicates, from reaching the recipients.
If you aren't comfortable with stuff like email headers and distro lists, Customer Support can help. Just send them the original — not forwarded — message as an attachment. For help with that, see Saving Emails to Your Hard Drive.
Although duplicates aren't the only thing that can cause delivery failures, it's easy to eliminate them as a cause. There are a couple things you can review to prevent duplicates and the resulting delivery failures. Here's what to look for and your next step:
- A recipient's email address was included more than once in your message. It can happen when you use distribution lists. Review lists to make sure you haven't included the same address more than once.
- A recipient gets one message from you and one from another address. It's usually your message resent by someone you carbon copied. It can also happen if you send a message to recipients and to forwarding addresses they set up. Try resending — not forwarding — the message to your intended recipient.
If you're familiar with email headers, header content that's completely identical to that of another message is a sure sign they're duplicates. The first place to look is the Message-ID: values. It's not the only thing to check, though. Look throughout the header for differences.
It's important to verify header content is identical. You might accidentally send messages that appear to be duplicates, but aren't. For example, you'll see false duplicates if you click to send a message, and then click to resend the message if your email application is slow. Another way is due to email clients, such as Microsoft Outlook®. They can create duplicates by automatically resending messages if they're delayed, for instance, by firewall settings.
For more places to troubleshoot, try searching our Help Center with terms from the error messages, such as 552.
