Question

When developing an application that sends out notification email messages, what are the best practices for

1. not getting flagged as a spammer by your hosting company. (Cover any of:)
   • best technique for not flooding a mail server
   • best mail server products, if you were to set up your own
   • sending messages as if from a specific user but still clearly from your application (to ensure complaints, etc come back to you) without breaking good email etiquette
   • any other lessons learned
2. not getting flagged as spam by the receiver’s client? (Cover any of:)
   • configuring and using sender-id, domain-keys, SPF, reverse-dns, etc to make sure your emails are properly identified
   • best SMTP header techniques to avoid getting flagged as spam when sending emails for users (for example, using Sender and From headers together)
   • any other lessons learned

An additional requirement: this application would be sending a single message to a single recipient based upon an event. So, techniques for sending the same messages to multiple recipients will not apply.

Answer 1

best technique for not flooding a mail server

not a lot you can do about this beyond checking with your mail server admin (if it’s a shared hosting account / not in your control). but if the requirement is one email to a single recipient per event, that shouldn’t be too much of an issue. the things that tend to clog mail systems are emails with hundreds (or more) of recipients.

if you have events firing off all the time, perhaps consider consolidating them and having an email sent that summarizes them periodically.

sending messages as if from a specific user but still clearly from your application (to ensure complaints, etc come back to you) without breaking good email etiquette

you can accomplish this by using the ‘Reply-To’ header, which will then have clients use that address instead of the From address when an email message is being composed.

you should also set the ‘Return-Path’ header of any email, as email without this will often get filtered off.

ex.

From: [e-mail] Return-Path: [e-mail] Reply-To: [e-mail] 

configuring and using sender-id, domain-keys, SPF, reverse-dns, etc to make sure your emails are properly identified

this is all highly dependent on how much ownership you have of your mail and DNS servers. spf/sender-id etc… are all DNS issues, so you would need to have access to DNS.

in your example this could present quite the problem. as you are setting mail to be from a specific user, that user would have to have SPF (for example) set in their DNS to allow your mail server as a valid sender. you can imagine how messy (if not outright impossible) this would get with a number of users with various domain names.

as for reverse DNS and the like, it really depends. most client ISP’s, etc… will just check to see that reverse DNS is set. (ie, 1.2.3.4 resolves to host.here.domain.com, even if host.here.domain.com doesn’t resolve back to 1.2.3.4). this is due to the amount of shared hosting out there (where mail servers will often report themselves as the client’s domain name, and not the real mail server).

there are a few stringent networks that require matching reverse DNS, but this requires that you have control over the mail server if it doesn’t match in the first place.

if you can be a bit more specific i may be able to provide a bit more advice, but generally, for people who need to send application mail, and don’t have a pile of control over their environment, i’d suggest the following:

• make sure to set a ‘Return-Path’
• it’s nice to add your app and abuse info as well in headers ie: ‘X-Mailer’ and ‘X-Abuse-To’ (these are custom headers, for informational purposes only really)
• make sure reverse DNS is set for the IP address of your outgoing mail server