Yes avast! email scanner will still work, even if the SMTP port is blocked, even if you scan outbound email, it wont get delivered as the ISP has blocked the port. Unless they have some other way of you being able to send email (web based email, must be on-line to compose and send, IMAP, but I doubt this).
Port 25 is only one of the ports that avast's email scanner monitors and this is your SMTP port, meaning you can't send emails. You receive email on port 110 (POP3) and that to is monitored by avast.
Are you saying that there are no other options, e.g. port 26, normally an ISP might block port 26 (the alternative SMTP port). Personally I wouldn't touch an ISP with this kind of restriction with a barge pole, you really can do without the hassle. Some email programs may have problems with the port settings if the ISP insists on using a different port to the default SMTP port 25 for sending email.