Check with your recipients

The easiest way to find out what happened to a lost email is to approach the problem from both sides. Look at your email settings and ask your recipients to do the same. Here are the first steps you should take:

• Check out your filters to find out if your email could have landed in a different folder.
  
• Check your email client's POP/IMAP configuration.
  
• The subscriber may have accidentally deleted the email. Ask them to take a look at their trash folder.
  
• Ask your subscriber if they have anti-spam software enabled.
  
• Check the spam folder. There is many a chance that your email have landed there.
  
• Make sure both the recipient and sender addresses are in each other's trusted contacts list.
  

Corporate filters

Things can be a bit trickier when working in the corporate domain. There are likely to be a lot more restrictions/hurdles before we reach the intended recipient's inbox. All the above possibilities apply here as well but there are additional cases to look out for as well.

Firewalls - Usually corporate networks have firewalls set up to filter out unsolicited emails. To make your emails pass the firewalls, get in touch with your corporate subscriber directly. 

POP and IMAP

Post Office Protocol (POP) and Internet Message Access Protocol (IMAP) are what allow you to download emails from a remote server. If you have not integrated your device with one of these protocols, then you may not receive email notifications. Go for the protocol that best suits you and integrate it with your devices.

Bounce cases

Email address validation issues can cause your emails to bounce. Make sure that there are no errors or defunct addresses in your mailing lists.

Quarantined zone

Your email may be quarantined by the recipient's email server. This can happen if your recipient's inbox is full, if you are sending from an unverified address, or if your domain has spam complaints against it. In these cases, you should try to get in contact with your subscriber about it.

Check back after a while 

It's easy to be impatient when you're sending out an exciting new campaign. You may be surprised to learn that when it comes to campaigns, emails may not be delivered instantaneously. Try checking on your campaign again later, you may find that it just needed some time.

Floods or drips

If you send messages either too frequently or infrequently, you run the risk of having emails sent to the spam folder. A good baseline is no more than two emails per week and no less than one per month.

Error codes

Temporary failure - 400 error

A temporary failure where the target mail server returns an SMTP error code between 400 and 499 means the mail server is temporarily unable to accept your email message. When you receive this error, the most likely case is that the recipient's server is busy. Usually, mail servers will try again for up to 5 days before giving up. After 4 hours, they usually send a notice back to the sender explaining why there is a delay. 

Permanent failure - 500 error

A permanent failure returns an SMTP error code between 500 and 599. This code means that the mail server will not accept delivery of this message and it should be returned to sender without further attempts to deliver. Some reasons for these errors include:

1. The recipient’s email address doesn’t exist.
   
2. The email server no longer hosts email from this address—e.g. their account has been suspended or closed, or the customer has moved to a new provider without properly updating the domains MX records
   
3. The sender misspelled their recipient’s email address
   
4. The recipient’s spam filters have denied the message
   
5. The sender’s mail server is blacklisted.
   

Permanent errors generate a delivery failure message that is sent back to the sender.

One Stop Solution

The best solution for every deliverability issue is to ask your subscribers to add you to their contact list. This will remove the chances of being stopped or blocked by most filters and anti-spam software.