After transferring from Telenor to Get as my internet service provider, I noticed that some of my mail accounts would not let any mail through anymore. As I discovered, there were no response from any SMTP mailserver but one, namely Gets own SMTP server. Telenor also blocks communication to other servers than their own over TCP port 25 (SMTP), but at least there is a choice to switch this extra filtering off via your user webpages. When contacting Get about the same feature, after a loong wait, they responded that they are blocking all of the following ports to their customers:
21 TCP FTP InASP above means Address Search Protocol, not Active Server Pages, in this list.
25 * TCP SMTP In and out (except Get)
69 UDP TFTP In and out
80 TCP WEB/http In
137-139 TCP/UDP NetBios In and out
161 UDP SNMP Out
548 TCP AFPoverTCP In
12345 ICMP/TCP Netbus In and out
27374 * TCP ASP In and out
It is an annoying fact that this info is not available on all ISP's webpages (e.g. under FAQ or a technical info section) that runs such a practice - you have to beg for it. If you have higher requirements than mainstream web browsing and mail via your ISP's own servers, it is better to check with your potential new ISP before moving your business there and find out later that you have not gotten the connection you need.
If you need to run your own web, SMTP or FTP service over standard ports (running them on unstandard ports are of course no problem), and you have an internet provider that does not think that your internet conection should be fully open and your own responsibility, you could use services like the free DynDNS to get traffic in and out.
Telenor's info on blocking/opening SMTP port 25 (Norwegian)
Get's only info on the subject - not much (Norwegian)