Do you mean outgoing or incoming SMTP? If they block outgoing, this effectively means you can no longer send any e-mail as all mailservers on the internet expect mail on port 25. There is no real way around this short of changing ISP.
If they block incoming SMTP (most ISPs tend to do that nowadays for their low-end subscriptions) you have four options:
1/ Get all mail forwarded to a POP3 mailbox and empty it with multidrop
2/ Use dyndns.org's Relay service to get the mail on a non-standard port.
3/ Change ISP.
4/ Talk to your ISP and/or take a more expensive subscription
Options 1 & 2 have their risks and disadvantages. Option 3 means you also have to change DNS, which can take about a week to work properly. I would therefore recommend to talk to your ISP before trying something else.
Michiel