The message has not been sorted into spam. If it is sorted into spam, an AutoResponse will be sent as soon as the message is sorted into a non-spam area (as of FogBugz 7.1.6), so long as the rest of the criteria is still met.
The address that the AutoResponse would be sent to does not contain the strings 'postmaster','daemon','owner-','bounced','no-reply', or 'do-not-reply' anywhere to the left of the @ sign.
The mail precedence of the received email isn't set to 'junk' or 'bulk'. Until FogBugz 7.1.9, the 'list' mail precedence was also filtered against.
The recipient has received fewer than three autoresponses in the last hour. This is to prevent infinite email loops. There is a feature request to make this number configurable.
And, of course, if AutoResponses are disabled, FogBugz will not send them.