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This article covers checking port 25 for possible blocks from your ISP.
Issue:
You are able If you are able to receive mail however you without issue, but are unable to send mail from your mail client and you are certain your outgoing mail server settings are correct, you might be blocked by your ISP from connecting on port 25. 

The Cause:
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:
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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.
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This is what you should now see:
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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 should see the following results:
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You are able to connect to the SMTP server through port 25. Your ISP is not blocking the connection.
However if you see the following:
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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 ISPdefault port for SMTP (sending mail) is port 25. Some ISPs block access to port 25 in order to try to avoid some issues with spammers. However, this can also block you from being able to connect to your mail server to send mail through your domain. Alternately, you may have port 25 blocked in your local router or cable modem. This article will explain how to check if port 25 is blocked from where ever you are and what you can do about it. 

How to Check if Port 25 is Blocked

Here is how you can check to see if port 25 is blocked. Follow the directions for the OS that you are using locally (not on the mail server itself).

What to Do if Port 25 is Blocked

1. Configure your mail client to use port 26 or port 587 for outgoing mail (SMTP) instead. This will usually do the trick.
   1. On a cPanel server, getting Exim to listen on another port is easy, just check Exim Mailserver on Another Port in the Service Manager feature in WHM and select the port to use (26 is the default), then save changes.
2. Contact your ISP to confirm they are blocking port 25. You'll likely need to change the SMTP port (as in step 1 above) to get around this. ISPs rarely open closed ports for non-business internet clients.
3. If your ISP says they are not blocking port 25 or 587 and you are still having problems sending mail, check your local router or cable modem to see if port 25 is blocked there.
4. If nothing else works, and you have service with HostDime, please contact us and we can investigate.