This article covers checking port 25 for possible blocks from your ISP.
You are able to receive mail however you are unable to send mail from your mail client.
Internet service providers (ISPs) are starting to block their customers from accessing port 25 on servers located elsewhere on the Internet. Port 25 is the default ports used by SMTP. SMTP stands for Simple Mail Transfer Protocol, and is the protocol used to send email to any outgoing email server. This port is used by all outgoing e-mail servers, and email programs which send email such as Microsoft Outlook and Mac Mail.
How to check if you ISP is blocking port 25 or 26:
Step 1: Open up command prompt.
How to open command prompt:
Windows Vista/XP/98: Start > Run > type cmd > press Enter
Windows 7: Click the Windows icon in the lower left and type cmd into the box. Hit enter or select cmd with your mouse.
Windows 8: Click the Windows icon in the lower left and type cmd. Hit enter or select cmd with your mouse.
Shortcut: Hit Windows Key+r and type cmd then press enter.
Step 2: type “telnet yourdomain.com 25” then hit enter.
*Be sure to change yourdomain.com to the domain you are trying to send mail from.
You are able to connect to the SMTP server through port 25. Your ISP is not blocking the connection.
Then your ISP is blocking port 25.
Step 3: Once you are determined the results. Type quit and hit enter
What you can do if port 25 is blocked?
1.) Configure your mail client to use port 26 or 587 for Outgoing Mail.
2.) Contact Support so we can ensure that everything is configured correctly on our end.
2.) Contact your ISP.