There are several different reasons as the why your email delivery has been delayed. Most of the time, it involves the remote mail server having trouble receiving the emails.
- Remote Mail Server Offline -
The email server that you are emailing, might be offline, or
being updated. You will be unable to send emails to this recipient(s) if
this is the case.
- Grey Listing
- Sometimes remote email servers have thresholds on how many emails it
will accept per hour from certain IP addresses. If this limit has been
reached or surpassed, the email will have to wait until the time limit
has been reached.
If you are experiencing the issues mentioned above, unfortunately not much can be done either party as the server is processing the information directly.