Do SMTP providers block IP address ranges associated with certain ISPs?

Last summer, my mother-in-law reported that she was unable to send email using Outlook Express.  I did a little research and found out that I had to configure Outlook Express on her machine to use a special SMTP server provided by her ISP.  At the time, I assumed that her ISP was blocking the traffic to her regular SMTP server.  

Did I have it backwards?  Is it the SMTP provider blocking traffic from IP addresses associated with her ISP in order to block spammers that are associated with her ISP?
jdanaAsked:
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KOTiSConnect With a Mentor Commented:
Most SMTP Servers require authentication before you can send e-mails. ISP SMTP Servers allow users from their local subnets (the ones that belong to them) to send mails without using authentication.
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jdanaAuthor Commented:
Sure, my mother-in-law's standard SMTP server requires authentication.  Why was she unable to access this server going through her ISP?  Was the ISP blocking traffic to the SMTP server or was the SMTP server blocking traffic from the ISP?
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jar3817Connect With a Mentor Commented:
If those ISPs have bad reputations, sure they do. Nobody likes spam and if an ISP is not doing anything to stop spam originating form their networks they can get blacklisted.

You're talking about 2 SMTP servers, one from her ISP and some other one, where is this other one? Is it also run by her ISP or by some other company?

Some ISPs (to combat spam) will stop any SMTP traffic from their residential customers. They require those customers to use their SMTP servers so they can control what is sent out and monitor activity. So in cases like this, the only way to send mail is to use the ISP's mail server. This is probably what is happening with your MIL.
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KOTiSConnect With a Mentor Commented:
Maybe your mother's ISP blocks access to external SMTP Server (probably not, but you never know) or could be possible that the other SMTP only allows access from certain IP addresses. If the other SMTP belongs to a different ISP, then probably that ISP only allows their clients to use their SMTP - this is a way to keep a client by making him/her not want to loose his/hers e-mail address...
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KOTiSConnect With a Mentor Commented:
You can also use MxToolbox (http://www.mxtoolbox.com/) to check the reputation of your ISP's SMTP Server. If your SMTP Server is blacklisted, then most other Servers will not accept messages to avoid SPAM.
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joshopkinsConnect With a Mentor Commented:
Most DNS blacklists have ISP's residential IP addresses blocked.  Most email servers use one or more DNS blacklists to reduce spam on their servers.

ISPs (not all) in the last few years started requiring end users to send email through their mail gateway to reduce the chances that their domain would be blacklisted.

Comcast does block all communication on port 25 to all mail servers but their own.

From her machine you could try to telnet on port 25 to the mail server.  If you can connect then I would check to make sure you have the correct authentication settings when sending an email.  If you can't then chances are that port 25 outbound is blocked.
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ajaykeralaCommented:
use a different smtp addresss. best to use the gmail smtp.
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jdanaAuthor Commented:
Great discussion.  Thanks!
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