This is because it is trying to index in a queryset, which does not support negative indexing. This should be handled as part of the input verification. Pinging @grungies1138 in case he wants to peek at it.

Most likely raised because someone typed @mail 0. The table that shows the list of messages is a 1-based index, but the in-memory list is 0-based. the -1 is to allow for selection of the list item based on it's position in the list. If the user is using a 0-based attempt to pull the message, then they will not get the right message, regardless of the number they enter. Best I can do/recommend is to catch then entrance of a 0 in the input and display a message. Unless someone else has a better idea.

Indeed to catch the input of a 0 on @mail is what I thought about - just lack the knowledge about the system to implement it.

This might work as a technique to get it in range: once the input string is converted to a number, to make sure the value is positive and an integer:
value = int(max(1, value))

Maybe something like:

                    try:
                        num = int(self.lhs) - 1
                        if num < 0:
                            raise ValueError
                        message = self.get_all_mail()[num]
                    except (ValueError, IndexError):
                        self.caller.msg("'%s' is not a valid mail id." % self.lhs)
                        return

That way you're using the same error message.

@grungies1138 Are you interested in resolving this?

I fixed the contrib with more elaborate index error checking. So this should not happen any more. Closing.