Emails not delivered

If emails you are sending are not being delivered, or you are not receiving emails from a specific sender, the sender may receive a bounce-back email from their mail server explaining why the message was rejected. In this case, the bounce-back message usually includes details explaining why the message was rejected.

When no bounce-back message is received, the email may not have actually been sent. If the email still appears in the sender's outbox, the problem is likely that their mail client is set to "work offline" or is having an error connecting to their mail server.

When an email appears to have been sent on the sender's side but is not received and no error is reported, the message was likely detected as spam and filtered out. In this case, the recipient may find the email in their Spam/Junk folder, and the headers in that email should explain why it was detected as spam. The recipient could also have a mail filter which is removing the messages. Otherwise, the mail provider's IT support could check their email logs to track what happened to an email from the recipient's address and the time the message was sent.

Undeliverable bounce-back messages

Bounceback messages typically have a subject starting like "Undeliverable: ", followed by the original message's subject. The message should begin with details of the reason for the bounce back, for example:

This message was created automatically by the mail system.

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:

>>> recipient@ncf.ca (Undelivered): 550 5.7.23 <recipient@ncf.ca>: Recipient address rejected: Message rejected due to: SPF fail - not authorized. Please see http://www.openspf.net/Why?s=mfrom;id=sender@somewhere.ca;ip=12.34.56.78;r=;

In this example, the problem is that the message was sent from a mail server (with IP 12.34.56.78) which is not allowed to send mail according the SPF record on the sender's domain name (i.e. the part of their address following @, "somewhere.ca" in the example).

SPF fail errors

Domain names (like "ncf.ca") can set an SPF record to restrict which servers are allowed to send from their addresses (like "name@ncf.ca"), which helps block messages from other servers spoofing their addresses.

SPF checking tools can be used to check if a server's IP is allowed by a domain name's SPF, or if there are problems with the SPF.

Links to "openspf.net" no longer work because the site is offline, but the details in the link can be checked using another SPF tool. For example using the details from the error above: https://mxtoolbox.com/SuperTool.aspx?action=spf%3asomewhere.ca%3a12.34.56.78&run=toolpage

The SPF record can also looked up directly, for example on the command line with "dig":

$ dig TXT +short ncf.ca
"v=spf1 ip4:104.160.252.0/24 ip6:2607:c280:1::/64 mx a:mx4.ncf.ca a:mx5.ncf.ca a:mail.ncf.ca -all"

SPF records are normally set as TXT records and begin with "v=spf1", followed by a list of allowed servers. The ending "-all" indicates that all other servers are not allowed.

Blacklist errors

Another common reason for mail to be rejected is if the sender's mail server is blacklisted which is the case for errors like "Connection originating from an IP address with a poor reputation.".

Other bounce-back reasons

The recipient could be out of space in their mailbox, their address may not exist (e.g. typo in address or mail account is disabled), or their mail server could be having problems. The reason is usually explained in the bounce-back, otherwise the mail provider's IT can investigate using the details in the bounce-back.