I’m pretty confused on this topic so sorry for mistakes but I’ll try to be as cleaner as possible.
Basically we have a mail marketing software that allow users to send campaigns via phpmail/phpmailer of our hosting/server as sending servers or connect to third parties services like Mailgun, Sendingrid etc via API.
We are talking of 1-5K mails per submission and most of them are long time subscribers, nothing exceptional.
Currently we have users that have verified their sending domains trough the software and use phpmail/own vps as sending domains while others decided to use eg Mailgun. But in the latter case they don’t verify their sending domains.
How really those services impact delivery n technical terms without domains verification?
Basically they have as verified account only our agency domain while users send with their domains.
TLDR: Phpmailer/own vps+verified sending domains perform better or worse than mailgun/top delivery services without domains verification in terms of deliverability?
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Answer
This is really too broad a question for SO, and the answer to almost all of it is “it depends”.
Spam filtering and deliverability is driven by recipient actions at most major ISPs (apart from gmail, which seems to use a random number generator). If your recipients make a point of reading and responding to your messages, they will go into the inbox no matter what you’re saying and how you send them. If you have a long history of doing this, so much the better. The problem is that this is a very difficult point to get to, and building up that reputation without taking measures like authentication via SPF and DKIM, double opt-in, obvious unsubscribe mechanisms, relevant content, reverse DNS matching, DMARC, etc. Those things can be painful to set up, and external ESPs can make it easier – there’s no reason why you can’t get mailgun to do proper authentication, and anyone sending that way should definitely take advantage of that, otherwise they’re just throwing away opportunities.