Is the SME at work in server-only mode or server-gateway mode? (If it's in server-gateway mode, the firewall rules will be handled automatically when you enable PPTP connections).
Does your home router allow PPTP pass-thru? If so, is it enabled? (PPTP is an odd beast...)
PPTP uses IPv4 Protocol number 47 for data traffic once authentication has been done using TCP port 1723. Protocol 47 is also referred to as the GRE protocol". GRE, as pointed out by leiw, is a
protocol and not a port. That is, it is at the same level in networking technology as 'TCP', 'UDP' and 'ICMP' (Here's a list of IPv4 Protocols:
http://en.wikipedia.org/wiki/List_of_IPv4_protocol_numbers).
So, while SMTP uses the TCP
protocol (protocol number 6) combined with
port 25, PPTP uses the GRE
protocol (protocol number 47). Very, very few routers provide a specific way to control the GRE protocol - what they do instead is tie it to TCP port 1723 - so a router with "PPTP pass-thru" support will know to allow GRE traffic on any connection that has already been established on TCP port 1723.
If your routers (at both ends!) don't specifically mention 'pptp pass-thru' then they probably don't support it.
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