SMTP Port 25 is blocked - how can I open it?

Posted on 2011-10-25
Last Modified: 2012-05-12
We have just installed a new Small Business Server 2008 with Exchange 2007 four weeks ago. It has been working fine for the last couple of weeks.

But two days ago, my internet dropped out.  My ISP suggested I reboot the router.  Hey presto my internet worked.

Now today I realise I haven't been receiving emails since then.

I phoned the ISP who opened port 25.  I can only assume they closed it.

Still no inbound email.  

But outbound email works.  Also website hosted in my office on 2nd webserver works as does FTP on my webserver.  Port 25 is open on my router.

Telnet 25 ... no connection

Someone please help me analyse this problem.
Question by:enigmasolutions
    LVL 10

    Expert Comment


    check with your ISP If they block SMTP traffic or not.
    LVL 49

    Expert Comment

    first I ran an smtp test on your server and yes port 25 looks blocked

    however i tried to send email to and noticed that is your relay.

    is your domain name ?
    LVL 1

    Accepted Solution

    Wholly crap!  I fixed the problem... this is how... (you will not believe it).

    About 7 years ago I was using Netgear FVS 318 routers, and they all had this strange problem where if you mucked around with the TCP/IP port mappings (eg adding/deleting/editing) then the router would randomly forget one of the port mappings.  To solve the problem you simply had to clear out all the entries and re-enter them.

    Guess what!  This Netgear bug is still around in their latest DG834 routers.

    Just now I managed to fix the problem by deleting the SMTP port 25 mapping and simply re-adding it (exactly as it was).  Totally illogical!
    LVL 1

    Author Comment


    FYI - I use trend micro's Hosted Email Messaging Service - which just filters our spam before it hits my server.  I was going to mention it.

    Anyway I am still astonished at the solution to this problem.
    LVL 1

    Author Closing Comment


    Featured Post

    How to run any project with ease

    Manage projects of all sizes how you want. Great for personal to-do lists, project milestones, team priorities and launch plans.
    - Combine task lists, docs, spreadsheets, and chat in one
    - View and edit from mobile/offline
    - Cut down on emails

    Join & Write a Comment

    Create high volume marketing opportunities using email signatures with these top 10 DOs and DON'Ts of email signature marketing.
    Resolve Outlook connectivity issues after moving mailbox to new Exchange 2016 server
    In this video we show how to create an email address policy in Exchange 2013. We show this process by using the Exchange Admin Center. Log into Exchange Admin Center.:  First we need to log into the Exchange Admin Center. Navigate to the Mail Flow…
    The video tutorial explains the basics of the Exchange server Database Availability groups. The components of this video include: 1. Automatic Failover 2. Failover Clustering 3. Active Manager

    728 members asked questions and received personalized solutions in the past 7 days.

    Join the community of 500,000 technology professionals and ask your questions.

    Join & Ask a Question

    Need Help in Real-Time?

    Connect with top rated Experts

    20 Experts available now in Live!

    Get 1:1 Help Now