Ask HN: What are less-known best practices for email deliverability?

Limit concurrent outbound SMTP streams to any single provider to something reasonable. Many interpret an excess as a spam signal.

Spend time interpreting bounces that come back as emails versus hard failures on SMTP delivery. This isn't easy. Either highly manual, or lots of parsing out temporary versus permanent failures, versus human emails, etc.

2 rules I follow: Don't get flagged as spam. Don't send messages to unvalidated email addresses.

Spam- Few things move faster than an annoyed user's mouse going for the "Report Spam" button. It should be 1 click, with links at the top and bottom of the message. Do it in real-time, not in "24-72 hours." No logging in to change communication preferences. Don't bury it in fine print or hide it with CSS.

Invalid Addresses- Some email providers penalize (by slowing delivery) the originating IP address every time that IP attempts to email an invalid address. If your mailing list software then retries delivery to a bad address, the penalty increases (exponentially with some providers). This can create denial/degradation of service vulnerability for your outbound mail.

Just from the user perspective, frequently legit E-mail goes into my spam folder. This happens with different major E-Mail providers. Although it's mail I'm not interested in, it gets flagged as spam and I unflag it if I find the time. But the point is, usually it's double-opted in mailings which is not interesting to me at all and received in much to high frequency.

So if you are writing a system for that from scratch, consider decreasing frequency for people who don't open the mail. So it passes through spam filters, and maybe raises attention of readers, just for curiosity...

To add my two cents: my company had the same problem - despite checking all the boxes ourselves.

When testing, something we noticed was that the content of our emails/newsletters had a significant impact on whether they ended up in the inbox vs. spam.

Of the major service providers, Outlook seems to put huge importance on the content of the email. The simple addition or deletion of a couple of words can dramatically affect spam rates.

Yahoo is less trigger-happy about content; and Gmail least of all. However, even for domains with a good sender reputation, Gmail will spam emails if the content is too 'spammy'.

Shameless plug here: we ended up building a tool to test out various email subjects/bodies against the big 3 providers above (you can even plug in your own email address/service when testing). Do try it out - I'm sure you'll find the results eye-opening: https://vetter.monsoonyeti.com/

It seems like you're doing many things right.

One over-looked trick: "warm up" the inbox you send from.

I.e. start sending mail at ever-increasing volumes until you've reached the max capacity you need to send at.

If you Google "Sendgrid Warm Up Schedule", there should be a PDF with a send schedule you can copy.

In my experience the better the picture(s) the higher the open rate. That is why art related newsletters have the highest open rate numbers. I worked at an art gallery once and we had a phenomenal 12% open rate for our emails which were purely informative.

Other than that, you should consider the option of clearing your recipient list every once a while. People who haven't opened a single email in a course of a year (or less) should be removed.

One trick that some e-commerce sites do is to send different emails to users depending on their purchasing history. Depending on your industry this may or may not apply.

> Excluding unengaged subscribers who have not opened an email from us in the last XXX days

How would you know if they've opened it? For example, I never allow the display of pictures (external ones, not included in the email itself via data URIs) in my email client, nor anything else that might result in a ping to a server. That's the default behavior of most email clients, isn't it?

Please send and obey 'Precedence:' headers.

What is the age of your domain registration?