While manually reviewing the stats.dat file within your mail server installation folder or using a custom script, you notice two variables, "mtaFailedSoft" and "mtaFailedHard," but it is unclear what they represent.
The variables within the stats.dat file are internal references for the various statistics items available from the Status > Statistics menu in WebAdmin. All of the "mta" items represent various Mail Transfer Agent statistics related to SMTP Delivery. The mtaFailedHard and mtaFailedSoft track the outgoing deliverability statistics (it is not for incoming mail to your server).
mtaFailedHard: Mail Transfer Agent Hard Failures - this is when your server cannot deliver mail to an address. It can happen when:
- A user sends to the wrong email address.
- A server is unreachable.
- Any other error is considered "permanent" where the mail server cannot deliver the mail.
- The SMTP protocol defines different error messages for "permanent" (5xx) and "temporary" (4xx) failures.
- See SMTP Conversations for more details on the server responses.
mtaFailedSoft: Mail Transfer Agent Soft Failures - this is when your server cannot deliver mail to an address temporarily. It can happen when:
- Mail is delayed, for example, when the remote server temporarily rejects your message for their Graylisting spam filter.
- The mail server will try again after a while.
- DNS resolution is temporarily unavailable.