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Why is My Mail Undeliverable?





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Here are some common reasons why emails are not delivered.

SPF is not enabled on your mailbox
What does this mean?
SPF is a DNS record designed to differentiate legitimate email from spam. Many mailboxes use SPF to either deter spammers from “spoofing” their email address, or ensuring that incoming mail is legitimate. Some popular email services even go so far as to automatically reject any email address that does not have an SPF record enabled. 

Is it happening to you?
This may be happening to you if you’ve received a bounceback error message that has mentioned that the mail is “unauthenticated” or that your message did not pass an SPF or DKIM test. 

How to Fix
If you suspect SPF requirements are the reason your emails aren’t being delivered, try enabling an SPF configuration on your account. 

Pair Networks offers pre-configured SPF records for our mailboxes so it’s easy for you to get SPF protection, without building an SPF record from the ground up. Find out more about our pre-configured SPF records here

If you’re comfortable with DNS records, you can also create your own SPF record.

Tip: You can also pair your SPF record with a DKIM record, which may improve email deliverability. See our DKIM article for more information. 
The recipient's inbox is full
What does this mean? 
If the recipient has reached the space limit of their inbox, their email may reject any incoming mail. 

Is it happening to you?
You can tell if this is the case with your email if you see, “Quota Exceeded” anywhere in your bounce back email. 

How to fix 
If this is the case, there isn’t a great way to fix it, unless you have another means of communication with the recipient so you can tell them to make some room in their inbox. 
The good news? Your email was most likely cached by their account. This means that if the recipient makes room their account in the next few days, there is a good chance that all the mail that was blocked will be delivered. And you won’t even have to lift a finger.
Your email is too big
What does this mean?
Does your email have a big attachment or a lot of visuals? If it does, your email probably exceeds the maximum size the receiving email could handle. Our email servers also have a size sending limit of 40MB.

Is it happening to you?
This may be the problem if you received a bounce back email mentioning exceeding the size limit. 

How to fix
You can fix this by slimming down your email. If you are sending pictures, reduce their size before uploading them to your email. If you exceeded the size by sending several attachments, consider sending them in multiple emails. This will help conserve space and ensure your email goes through.

If you are trying to transfer files between machines via email, try using scp or SFTP. The email protocol SMTP (Simple Mail Transfer Protocol) was not designed to transfer large files, but Secure Copy Protocol (SCP) or Secure File Transfer Protocol (SFTP) was designed for large file transfers.
You have gone over your sending limit
What does this mean?
Pair Networks servers do not permit sending out mail to more than 25 addresses in one batch, whether sequentially or in parallel. If you try to send emails to more than 100 recipients in one batch, you may be temporarily locked out of your server.

Is it happening to you?
This may be the case if you received a bounceback email stating that your previous email exceeded the set “limit.” 

If you have been locked out server after attempting to send out 100 or more emails, it is very likely that you have gone over your sending limit. 

How to fix
Consider using a mailing list service to do these kinds of mass mailings.

If you are locked out of your server for trying to send too many emails, contact Pair Network’s support.
The recipient email doesn't exist
What does this mean? 
There may be a typo in the email or the person may have recently deleted their account.

Is it happening to you?
It should tell you in the email that delivery could not be completed because the recipient was not found.

How to fix
Check to see if there is a typo in the address. If you are sure there isn’t, see if you have another means of reaching out to the recipient to verify their address.
The receiving server is overloaded or unavailable
What does this mean?
Sometimes servers go down for maintenance or traffic overwhelms them. If this is the case, the email might bounce back to you. 

Is it happening to you?
This will cause a delay between sending and reception. If the bounce-back email says that the email was “undeliverable,” this could be the cause.  

How to fix
Don’t worry. Most of the time this is temporary. Most email servers will continue trying to send the email for a couple of days. When the traffic decreases or the server is relaunched, your email will be delivered. 

However, if someone has permanently taken down their server, you may also get the same message in the bounceback email. If you are still receiving the same bounce-back message after a few weeks, you may want to remove the address from your list or check in with the owners if you have another means of communication.
Your emails are 'greylisted'
What does this mean? 
Greylisting looks at incoming mail’s IP address and, if it doesn’t immediately recognize it, the email will be temporarily rejected.
For more details, check out this article.

Is it happening to you?
If this is happening to you, it may be causing a delay between when you sent out the mail and when the recipient receives it. 

How to fix
If you are sending out legitimate emails, this problem should fix itself. The greylist only rejects an email once since junk emails rarely resend their emails. Legitimate emails automatically resend emails that have been greylisted, so your email should be delivered after a short delay. 
Once an email address has gone through the greylisting process, it should not be subjected to it again for a 30-day period. 

A way to guarantee this doesn’t happen again is to contact the recipient to ask them to add your email to their “Accepted Addresses” list. Any address on this list will not be greylisted.
Your server's IP address is blacklisted or blocked
What does this mean? 
This means that your server has been marked as a possible source of spam. Blacklisting is a service that monitors incoming mail and blocks junk mail. If your email address has been flagged as spam by multiple 

Is it happening to you?
Bounceback messages of this sort may say that your IP address has been blocked. A URL may be provided in the bounce-back email for more information. 

How to fix
If you have been blacklisted, there are several things you can do:

- You can wait until your name is taken off the list. This is usually a temporary problem (unless you are actually sending out spam), so one option is to just wait it out. 

- You can put in a request for removal to the service that blacklisted you. 

- If you have another means of contacting the recipient, you can request that they add your email address to their “Accepted Addresses”

If you have been blocked by the recipient, however, there’s not much you can do.

Note: Sending out spam is in violation of Pair Networks’ policies and could result in account termination. To see more about this policy, click here.

Contact Support

If none of these reasons fit your situation, contact Pair Network Support:

Updated on April 8, 2024

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