Most ISPs block port 25 so that home users' computers, if compromised by a virus or something, cannot become a source of spam. You may want to check with your ISP about what they do and do not block.
Also, what that really does is prevent you from sending mail through your ISP's mail servers that does not originate from within your ISP's network.
Without knowing what syntax you're using for sendmail, it's tough to tell exactly what the problem could be. You need to post more information in order for us to troubleshoot the problem -- in other words, "help us help you." You say you can't send mail, but you don't explain what's going on... does the sendmail program just quit without notice? Does it give you any error messages? Do the messages appear to send, but really don't send?
Please be much, much, much more specific. It's better to give us too MUCH information than too LITTLE. Explain the steps you're taking, post the exact sendmail syntax you're using, and tell us any and all error messages you receive.