In theory I don't see why not, but it probably isn't a good idea.  Doing that way makes the spamsnake a bridge between the two networks that isn't protected by the firewall.  The only way to make it secure is to have the firewall accept mail on the external interface, with the appropriate firewall blocking, pass it to the spamsnake on the internal interface for processing and then forward it to a mail server for distribution.  If the spamsnake accepts mail on the external interface directly, it will bypass the firewall.

The more secure option is to have the spamsnake be a separate external machine, accept and process all mail there and only pass the legitimate mail to the internal network via the external interface of the firewall.  The other benefit of this method is it reduces the load on the firewall since all the spam, and the associated connections, has been dumped before it reaches the firewall.