Emails not delivered
If emails you are sending are not being delivered, or you are not receiving emails from a specific sender, an error message may be sent back from the mail server explaining why the message was rejected. In this case, the bounce-back should explain 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 outbox, the problem is likely that the mail client is set to "work offline" or is having an error connecting to the 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 the 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 can check their email logs to track what happened to an email by looking up the recipient/sender address and the time when the message was sent.
Messages are often blocked because an attachment is too big or is detected as malware, or if something in the subject/content appears to be malicious. Sending a simple test email can identify if the problem has to do with a specific message.
Undeliverable bounce-back messages
Bounce-back messages typically have a subject starting like "Undeliverable: ", followed by the original message's subject. The message should begin with details explaining why the message was rejected, 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 (e.g. "ncf.ca") can set an SPF record to restrict which servers are allowed to send from their addresses (e.g. "name@ncf.ca"), which helps block messages sent from other servers spoofing their addresses.
SPF 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 this checks the details from the error above:
https://mxtoolbox.com/SuperTool.aspx?action=spf%3asomewhere.ca%3a12.34.56.78&run=toolpage
SPF records can also be checked manually, for example on the command line using "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.
Looking up the server's IP address via Whois/RDAP to see who it's registered to may help explain what changed to cause the error. For example, the sender may have changed which service provider they use for sending emails, and need to update their SPF to include the new provider's servers.
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, account disabled), or their mail server or domain name 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.
Testing bounce-back issues affecting all addresses on the same domain
If messages to an address such as "recipient@somewhere.ca" are bouncing, a test can be sent to a fake address on the same domain such as "test@somewhere.ca" to check if it's a general problem with the server/domain (same bounce-back error), or specific to the account (the fake address may have a different error saying the address does not exist).