Deliverability is a big marketing/selling point for all these providers & important selection criteria for their customers. So it'd be very interesting to see what % of emails for each provider landed in Inbox vs Promotions/Spam/Junk folder.
Are those percentages really a good measure of deliverability? I mean, a significant portion of “one million dollars for you” scams in my Gmail spam folder are from @gmail.com addresses; that doesn’t say anything about gmail.com to gmail.com deliverability.
I was thinking about spam as I read the post too, since this is a problem I get to deal with almost daily. Marketo has a terrible signal-to-noise ratio and it can often end up blackholed on my network with nary a user noticing. Sendgrid is my next least-favorite email-as-a-service, and in fact I happened to have this article open in one tab and in another tab I had this morning's automated reports from some of my spam software showing that a pile of inbound messages from Sendgrid had tried to land at a list of nonexistent mailboxes -- someone was rolling through "andy@", "barbara@", "vijay@", "kenneth@", etc.
I have had very little trouble with Mailgun in this regard, so far, but that may only be because the spammers haven't fully adopted them yet.
As a sysadmin on the receiving side, I really loathe these services, they make my job significantly harder and more time consuming than it used to be.
Unless I misunderstood, the person you replied to literally says he has emails from @gmail.com throwing in his Gmail spam filter. So not sure why you mention Gmail is special here (regardless if it's true or not).
If you mess-up really bad you will have bad deliverability across the board.
The mistakes can be numerous:
* bad setup like broken DKIM
* bad sending parameters like opening too many connections by IP to a given provider
* bad content format like too many large images
* bad actual content
* bad recipient list management with lists containing lots of typos, none existent mailboxes, spam traps, etc
* miss-managing unsubscribes
* bad bounce handling
If this happens, you can have some if not all of your IPs blacklisted, maybe even your domain. In worst cases, a whole IPv4 range is blacklisted.
Then a second aspect to take into account is the open rate and the click through rate (people actually clicking a link in a marketing email), this one can vary depending on the company activities, and also the quality of their marketing content, but is generally in the range of a few percents at most. And then, you also have to take into account the actual impact in term of sells which will be even lower.
Deliverability is import, it's not rocket science, but it's a lot of small items various people at various stages (tech, creatives, marketers, etc) must be cautious about which makes it actually quite hard.
Maybe the other big operators are more content-based?
What you say certainly rings true about Gmail - email subjects/bodies that set off warning bells in Outlook seemingly pass through Gmail without a hitch if your account has a good sender reputation - and vice-versa. However, we've also noticed that even if you have a good sender reputation, Gmail can and will (automatically) spam your content if the wording is too spammy/phishy.
Yahoo seems to be midway between Outlook and Gmail, at least from what I've observed.
Anyway, do give the tool a spin and try out various types of emails. I'm sure you'll find the results interesting!
[EDIT] Oh, BTW - you can set the tool to use your own service as well! Obviously, this will be more accurate than using the default (which is an SES account we use).