IIS SMTP used to relay Contact Us form messages to Gmail has been blacklisted by Google

Question

I have 2 Windows 2008 R2 boxes running in Microsoft Azure. My ASP.NET 4.0 site (let's imagine it's running at "example.com") has a standard Contact Us form.

When a user sends a Contact Us message, I use System.Net.Mail and SmtpDeliveryMethod.Network to deliver mail to an IIS6 SMTP server running on each box, which sends the mail to a Google Apps "enquiries@example.com" account, using the email address the user entered into the Contact Us form as the "From" address.

This was working beautifully for a year until I checked it today, and found this error in a .BDP file in the \Badmail folder:

550-5.7.1 Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been blocked. Please visit http://www.google.com/mail/help/bulk_mail.html to review our Bulk Email Senders Guidelines.

Obviously Google upped their anti-spam strategies in the last 6 months - last time it worked was Feb 2013 (yeah, we don't get much mail luckily... yet).

I've read the Bulk Senders Guidelines linked above, but they're not really suited to my use case. My case is not sending emails from our server to users of our site (I simply use the Gmail API and send from our enquiries@example.com for that), but rather to collect users' enquiries so that we can easily respond by clicking Reply in that inbox.

I am looking for the easiest solution here. In response to the ones in Google's Bulk Senders Guidelines:

1. Use a consistent IP address to send bulk mail: I already do, doesn't seem to help

2. Reverse DNS: Godaddy, my domain and DNS provider doesn't seem to support them: http://support.godaddy.com/groups/domains-management-and-services/forum/topic/how-do-i-setup-reverse-dns/ Anyone know if there's a way?

3. Use the same address in the 'From:' header on every bulk mail you send: This is totally not my use case. I'll have different From headers in every email

4. SPF record: I think this only works if I am sending From ...@example.com every time. Is that right? My feeling is SPF doesn't help me here. Would love someone to enlighten me.

5. DKIM: This looks hellishly complicated, but I'll pursue it if someone thinks it can work in this case. Specifically is it OK that the From address doesn't match the "signing domain"? Anyone got any good "how to" links? And will this be sufficient for Google to un-blacklist me?

6. Sendgrid: Azure's preferred mail sending app. This means signing up, code changes, testing, and unknowns like "does Sendgrid allow any From address? It's non-trivial, and I'd like to avoid this, but again, will go there if it's what people think is the sanest option.

Answer 1

As a general answer to your questions, sending email on behalf of many different domains from one IP (e.g. example.net, example.org, and ex.co from 10.0.0.1) is generally seen as spammy behavior (and therefor not recommended).

Your points 1-5 only apply if you're sending from one domain. rDNS, SPF, and DKIM only improve delivery for one IP to one domain (in a generally 1:1) relationship.

Generally, the best way to avoid getting marked as spam in a situation like this is to set the From email as a consistent one that you actually control (e.g. enquery-sender@example.com), and then setting the Reply-To as the entered address (e.g. enquirer@someprovider.com). This way you consistently send from one domain, while still getting the benefit of replies going to the message originator (for example LinkedIn does it this way). Doing this will allow you to setup rDNS, SPF, and DKIM with benefit.

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That said, if you decide that you don't want to use the recommended Reply-To method, you can use SendGrid to send from any arbitrary domain. It should not require any significant code change (just switching your current SMTP credentials to SendGrid's).

Disclaimer: I am a SendGrid employee.