What is the best way to handle email distribution across many applications?

Question

I have many applications across an enterprise environment and they all use different methods of sending emails. Some send directly through an exchange server, some queue up locally in an SMTP queue and others call aa web service that then sends the email.

I'm trying to decide on the best way to get guaranteed delivery of emails. If our Exchange server goes down, then the applications that send to it directly can no longer send emails, also any emails sent during the down time never get anywhere. I would also like to implement a universal templating solution that all applications can share.

Are there any pre-built solutions to this problem, or do you have an insight on how to handle this issue?

Answer 1

We solved this by creating a web service that sends all our emails. This web service uses the

System.Net.Mail.SmtpDeliveryMethod.PickupDirectoryFromIis 

setting, which essentially saves the files to a spot on the disk, tries to send them via the main SMTP server, and if the server is unavailable, they sit in the directory until it BECOMES available.

Guaranteed delivery, as long as the web service is up. Since we have redundancy checks in place, this is almost never an issue. If it is, we treat it as an error in code and handle it.

edit - added

I forgot to mention that XSS is a concern even in an email, so be sure to use something like the Microsoft.Security.AntiXss library, which contains functions like GetSafeHtmlFragment to strip out potentially dangerous scripts before outputting html to an email.

http://msdn.microsoft.com/en-us/security/aa973814.aspx

http://msdn.microsoft.com/en-us/library/aa973813.aspx

Answer 2

I have heard good feedback about Postmark. Maybe a service like that could be solution as it has several integration points.

http://postmarkapp.com

Answer 3

We have used HMailServer (on windows platform) which is freeware. Configured the max retries to too many & used external smtp to relay the emails. our applications ques up emails on the HMailServer and that server relays it further. with the max retries configured to be many if at all the main smtp servers are down we can assure that the email are delivered - but though there is no gurantee if there is huge downtime with main smtp relay servers.

Answer 4

I do this by queuing email to a SQL server database. Any ACID compliant database will work or you can use MongoDB with 'safe mode' inserts but if you really need guaranteed then use SQL server or MySQL. This way if your mail gets into the database it is 'guaranteed' not to be lost and your app doesn't have to think about it. You could use a web service or just make a shared assembly with a static public method in a class to drop your email in the db for you.

Include a column for status like 'new', 'delivered', 'recipient mailbox temporarily full', which you can represent with numeric values and keep a TimeToSend column, which starts out as the time when the email is queued in the database.

Then you have a mail app, that you can have run once a minute as a windows scheduled task. Make it as a console app. When it loads, it checks if an instance of it already running and if there is one, it exits. When running: 1. Attempts to deliver each mail to mail server. Query the database for all mail where the TimeToSend is older than now.
2. If mail is delivered to mail server, mark it logical deleted. 3. If any mail can't be delivered, advance the TimeToSend column for them to 10 minutes from now.
4. Delete records from table that are logically deleted. You can do this in the app or you can do it by having a sql job do it.

Answer 5

As mentioned earlier, you can utilize a web service that you can usually POST JSON to using an HTTP request. Here are a bunch of choices:

• PostageApp (Ours!)
• SendGrid
• PostmarkApp
• Amazon SES

They all have different feature sets and offerings, so definitely give them all a spin and figure out which you prefer.

( Full Disclosure: I am the Product Manager of PostageApp.)