In an effort to help combat SPAM, increasing numbers of ISPs both locally and internationally are implementing "Port 25 blocking", which restricts the use of external SMTP Servers.
If you're suddenly unable to send email from your HostUpon-hosted mail account, but can still receive email and access your website without difficulty, this is likely to be the cause. Typically you'll see the following error message, or similar - "Outlook unable to connect to outgoing server"
How do I fix the problem?
Within your email software, find the the account settings for the mail account you're having trouble with. You should see a setting for "Outgoing Server", "SMTP Server" or similar.
Change the default port from 25 to 250, apply the chnages and restart your email client.