Outlook cannot connect to your mail server ?

When setting up Outlook, or after ISP network changes, it is a common annoyance to be unable to send/receive email with a message of this type:

"Outlook cannot connect to your outgoing (SMTP) e-mail server. If you continue to receive this message, contact your server administrator or internet service provider (ISP)."

Navigate to your email account settings in Outlook under: Tools > Account settings > highlight your account and click "change" > click "more settings"

Look through all the tabs. Make sure the outgoing/incoming mail servers, your username and password are correct.

Under "Outgoing Server", make sure you check: "My outgoing server (SMTP) requires authentication" and "Use same settings as my incoming mail server"

In the "Advanced" tab:
Select the correct type of encryption, and the correct ports for "Incoming server" (usually 110) and "Outgoing server (SMTP)" - 25 is the default outgoing port, however, many major ISPs block this port to reduce outgoing spam, so if you're not using your ISP for email, it may be blocked and that is probably the culprit. As an alternative, try port 2525, or one of the secure ports below.

Here is a list of the default standard mail ports for your reference:

SMTP - port 25
POP3 - port 110
POP3S (POP3 over SSL, SSL-POP) - port 995
IMAP - port 143
IMAPS (IMAP4 over SSL) - port 993
SMTP mail submission - port 587
SMTPS (Secure SMTP, SSMTP) - port 465

You can also try scanning the above ports to see which one is open on your mail server using our portscan, and/or you can also contact your ISP to verify whether they are blocking any mail ports, port 25 is the most common one being blocked.

Instead of a portscan, you can also try testing if a port is open on a specific host by doing the following:

1. Open Command Prompt (Start > Run > type: cmd)
2. type: telnet hostname 25 (where hostname is the exact name of the SMTP server, and "25" is the port you'd like to test)

If your ISP (or your firewall) is blocking the port, you will not be able to connect. If you can connect to the port, it indicates some other Outlook configuration issue, even though it can still be a firewall/anti-virus software problem.

Notes:
Residential ISPs often block outgoing mail ports for outside mail servers (that don't belong to that provider), like ports 25, 465, 587. Some mail servers are configured to also use port 2525 as an alternative to 25 unofficially.
It is also possible that your software/hardware firewall is configured to block some of the above ports, reconfiguring it or turning it off temporarily may help troubleshoot.