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Ian Daz February 2016

.NET MVC Custom Mail Server

I'm in the process of planning the development of a mail-server to hand the sending of email across our multiple websites. Below is a description of what we are planning to implement and I'd like your opinion/suggestions.

We use ASP.NET MVC and have many web-sites hosted on Azure. We currently send mail internally within each of the web applications using SMTPServer.Send(). Obviously this is not the ideal way to send emails when you have a decently busy set of websites because the send mail call is blocking and cannot guarantee mails are sent. With this I'm worried out getting an influx of mail requests when we launch our next website (we think it'll get a decent amount of traffic and lots of emails will be sent).

My plan of action : to build a centralised mail-server that runs in the background (we use azure and this will be simply another web-application). When each one of our web applications wants to send a mail, instead of doing this internally, it'll call a web method on the mail-server called sendMail() this function will accept certain parameters and insert the mail parameters, content etc. into a database. The mail server will then poll the database at fixed time intervals, select a set of unsent emails and attempt to send them using the same SMTPServer.Send() function. If an email fails for some reason we won't flag it as sent and in the next poll interval the email will be selected again and another send attempt will be made. (we will cap the number of send attempts to say 20).

This will allow each of the websites to run smoothly without having loads of blocking send mail calls internally and the mail server will handle all the sending sequentially and in a controlled environment as a separate standalone web-application. Thanks in advance!


Chibi Chakaravarthi February 2016

Looks like a good design, Don't know the entire scenario which let to you building something like an email server. The problem has been solved well by using a service that already exist like Office 365.

Your design is good, My suggestions would be the following,

You can use Azure WebJobs to build the polling agent. You can make the web job run as a scheduled web job that does the polling and sending the mail and it can be written very clean as a simple console app.

You can use Azure API App to build the SendMail() call and you can use the Azure AD Auth on the API to authenticate the caller of the API using the Authentication and Authorization feature to easily secure your email server. You can also enable CORS easily as well to make sure you receive requests from other websites and process it.

Some issues I foresee for you,

  1. Volume and Scaling : You can only process so much email between each polling. If you cannot then you will need to create another polling agent which will making things complicated as they need to know they are picking different sets of emails to send. If you volume is going to be low you should be fine.
  2. Challenge : Why can't the websites send the mail them selves ? And then record it on the database for tracking. All you have to do build module or a component that they use on their web page to create and send the mail. Polymer 1.0 works well for this scenario.

Hope this helps to get you started.

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Asked in February 2016
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