Sure, in a nutshell, there's an issue with the mail server and its not available (SERVNOTAVAIL). Mail::Sender is noticing the unavailabilty, setting an error code, and then closing the socket to the server.

At its heart Mail::Sender uses SMTP. The protocol is quite simplistic (hence the S) and returns a code with each reply.

The author has decided to reduce the myriad of 4xx and 5xx STMP error replies to a single SERVNOTAVAIL message. In addition, if for some reason he cannot read from the socket (lost connection, crashed machine), he flags that as SERVNOTAVAIL too.

This is one of the disadvantages of using SMTP directly as opposed to an MTA (mail transfer agent) like sendmail. If you drop an email into an MTA, it may try to send the email several times for you. When using STMP directly, you have to decide what to do (drop them to the floor, queue them up for another go-round, etc).

-derby