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SMTP redundancies: How do I build them into my apps?

Posted on 2008-06-23
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Last Modified: 2013-11-30
SMTP mail services are a very important part of our web applications.  We run ASP.NET C# and Classic ASP apps.

I would like to know several ways to make these services redundant such that if one server goes down the other servers will still send content.

I have configured DNS, DCs, and AD on our systems, and I speculate that perhaps an abstract name can be used for SMTP services provided by multiple servers. If this is the case, how can I set it up using the MSFT DNS console?

Is there a way to do this in programming, both in Classic ASP VBScript, and in C#/.NET? In programming, there would need to be some test of SMTP connectivity followed by the attempt to send. Can someone post or link to a code sample?

Any other methods of accomplishing the same work?

Thank you,
-KF
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Question by:kennethfine
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Expert Comment

by:mastertechnc
ID: 21851447
The easiest thing is to setup redundancy in your code to connect to backup smtp servers.
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Author Comment

by:kennethfine
ID: 21851471
Thanks. Can you tell me how to do that?  
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carlnorrbom earned 500 total points
ID: 21866416
Hi,

I guess a rather simple (but still working) method would be to simply add all your host and the matching credentials into an array or object and simply loop through and exit on first successful transmission?!? Review the attached code snippet for a workable lightweight solution (tried it quickly on my dev pc. Added two hosts, my home smtp server and one in my office. Then ran the web app, first on home network then i connected using vpn mid through and it worked. It keeps looping if unsuccessful and breaks the loop when successful. You should be able to add any number hosts.)

The code snippet is in VB, I'm not a C# guy but I hope You get the general idea. You will need to redimension the hosts Object to match the number of SMTP Hosts you want to use.
'You will need to import System.Net.Mail (using in C#?!?)

Imports System.Net.Mail

'Dimension an Object or array to hold the smtp hosts and credentials

Private hosts(1, 2) As Object
 

        'Populate the SMTP Hosts and credentials

        hosts(0, 0) = "SMTP-Host-1"

        hosts(0, 1) = "User-Host-1"

        hosts(0, 2) = "Pass-Host-1"

        hosts(1, 0) = "SMTP-Host-2"

        hosts(1, 1) = "User-Host-2"

        hosts(1, 2) = "Pass-Host-2"

        'The logic to build message, instantiate smtp client and loop through hosts

        Dim m As New MailMessage()

        m.From = New MailAddress("sender@yourdomain.com") 'From Your business logic

        m.To.Add("recipient@somedomain.com") 'From Your business logic

        m.Subject = "The Subject Of Your Email" 'From Your business logic

        m.Body = "The Content Of Your Email" 'From Your business logic

        m.IsBodyHtml = True

        m.Priority = MailPriority.High

        Dim c As New SmtpClient()

        c.Timeout = 1000 'Timeout of Your choice depending on Your infrastructure

        Dim i As Integer = 0

        Do While i <= UBound(hosts)

            c.Host = hosts(i, 0).ToString

            Dim creds As New System.Net.NetworkCredential(hosts(i, 1).ToString, hosts(i, 2).ToString)

            c.Credentials = creds

            Try

                c.Send(m)

                'The transmission was successful, exit the loop through hosts.

                Exit Do

            Catch ex As Exception

                'The transmission was not successful, exit try and continue to loop through hosts.

                Exit Try

            End Try

            i += 1

        Loop

        m.Dispose()

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by:kennethfine
ID: 21868056
Thanks, good solution. I appreciate the help!
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