The following are some of the most common reasons why E-mails cannot be sent successfully.
Receiving mail server is busy.
The network is busy, resulting in receiving timeout.
Unstable network connection or slow network speed may also cause failure of E-mail sending, resulting in E-mail return.
Invalid mailbox or no permission
DNS resolution is invalid so can't get the server address of the other mailbox.
Your sender's mailbox is full or not enough remaining space or it is identified as invalid or listed in the black list of the other party.
The other mail server recognizes the sending SMTP server is not legal.
