You can use port 465 instead. To connect to that port, activate SSL in the email client.
Change the outgoing port number from 25 to 465 and select the checkbox "This server requires an encrypted SSL connection (SSL)".
For MS Outlook:
Tools > Email Accounts > View or change existing account
select the email account and press Change button
Click on More Settings...
Click on Outgoing Server tab
select "My outgoing server (SMTP) requires authentication"
and keep (o) Use same settings as my incoming mail server
Click on Advanced tab
under Outgoing server (SMTP)
select "This server requires an encrypted connection (SSL)
and change 25 to 465 in front of it.
Press OK and save those settings.