[CodeWriter23]

My experience is anything less than SPF, DKIM and DMARC results in Gmail silently dropping messages. Also, HTML mail with inline images similarly causes Gmail to drop.

And forget about sending from a DO Droplet due to poor IP reputation. A friend of mine at an ISP confirmed their third party SPAM scoring provider (like Symantec/ Brightworks) gives them a list of IPs to block at the edge, which includes large swaths of DO’s IP blocks. Not sure what the story is for other low-end VPS providers like Linode, etc.

Testing deliverability has become completely absurd. Every ISP and ESP has a completely different method for mail rejection. And thanks to outsourcing options for SPAM scoring, is subject to change at a moment’s notice.

This is a glaring omission in all Net Neutrality discussions I’ve seen, the ability to simply communicate an order acknowledgement to a customer. Even outsourcing to Mailgun, SendGrid or the like is no silver bullet for this issue, you’ll still have interruptions in delivery because every day some spammer gets through their checks and trashes that shared IP you’re sending from. And probably you’re having unnoticed delivery failures because someone else reported it / got it fixed. Renting a dedicated IP isn’t really a solution because you have to send from IPv4 to reach the guy who is still doing email @hisdomain.com and doesn’t even know IPv6 is a thing. And IPv4 addresses are in short supply.