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What Does it Mean When My Email Message Bounces?

A frequent question we get from our customers is “Why did my mail bounce?”

They refer to a situation when they send an email message, and after a few moments there’s an automated reply.  Usually from “Mail Delivery Subsystem” or a similar system information message.

Before I get into what those things mean, a quick look at how email works.

Email: Greased Lightning

Basic Email Communications

Mail doesn’t just go from your computer to the recipient in one connection.

Mail goes from your computer to the mail server, through a firewall, across the internet (where there can be 10 or more stops in alone), then through the recipient’s firewall, their server, and finally their workstation.

There can be even more stops with routers and other mail filtering systems, but I’m just showing the basics here.  Despite all these stops, everything typically happens in a matter of seconds.

The Process

The sending workstation passes it to the sending mail server, where it should be checked to see if you are a valid sender.  It passes through the sender’s firewall and on to the Internet.

The receiving firewall can (if it is configured to) check mail for spam, validate a legitimate sender, checks the mail for viruses, and after filtering, and passes it to the receiving mail server.

The receiving mail server can (if it is configured to) check mail for spam, validate a legitimate sender, checks the mail for viruses, and after filtering, and passes it to the recipient’s computer.

Here are some common scenarios when mail gets bounced:

Receiving Server Bounces

If the sender is determined by the receiving server to be “fake” then mail can be denied.  “Fake” would mean that the sending server is pretending to be something it isn’t.  Sometimes these messages bounce, but many times they don’t, to prevent a spammer from knowing if they were successful or not.

If the receiving mail server (after it’s gone across the internet) finds that the recipient doesn’t exist (or is misspelled for example), it will bounce the mail.  You may or may not get bounces for this, since many spammers will just guess at an email address once they know the domain name.

Mail Bounced For Spam

There are a multitude of reasons for this.  Here are a few:

  •  You use a 3rd party mail host (in other words you don’t own your own mail server on-premises), and one of the outbound mail connections was “blacklisted” because another client spammed from the same host.  Your outbound mail shares the same connections and can occasionally be tagged as spam.
  • The content of your email message is likely “scored” as spam.  Every email message has a “score” for spam.  No subject line increases the score.  Having an image or attachment with no message included explaining it increases the score.  Not identifying a sender increases the score.  Avoid this by always putting a subject line, explain any attachments included, and don’t clear out your “from” field.
  • Your mail server is not “registered” on the internet.  Many mail servers will validate the sending server by performing a “reverse lookup”.  Much like our telephone system, you can use caller ID to identify the caller.  Mail servers can cross reference the incoming communication against who it should be coming from in the “from” field and weed out spammers.

Message Too Big

Most servers have limitations on what size of emails can be sent or received.  These bounces can come from your own mail server if you have a limit on outbound mail size.  Generally speaking, you want to keep your email attachments under 10 MB in size (no more than 5 high quality photos, a document under 300 pages, etc.).

Mail Delay

Sometimes the bounce message isn’t a bounce at all.  At times, there is congestion on the internet.  If your sending mail server cannot send a message after 2-3 tries*, it will wait 4 hours* to try again.  The delay message is sent to you as a warning that your message is queued for delivery and may not try again for several hours*.

Your mail server will continue to attempt to send the mail for 2 days*, and if still fails, will then bounce the message back to you.

*These are configurable settings; typical defaults are given.

Email isn’t perfect…

Over 140 Billion emails are sent globally every day.  Arguably, over 60% of that is spam.  Despite that, the majority of our mail is sent on time, and error free.

Cloud-based email services like Office 365 centralize a lot of communication and help ensure reliable delivery and receipt of email.

If your electronic message is important, it’s always a good practice to just follow it up with a quick phone call.

To learn more about how email works for your organization or get your bounced email messages analyzed to determine what’s going on, you can contact Stewart and Son!  Call us today!

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