> of course you will be able to, but your e-mails won't reach most customers
Unfortunately, I found this to already be somewhat of a reality. Even with all the best practices I went to, many MTAs simply don't trust my IP address. Most "sane" MTAs seem to greylist me rather than blacklist outright, so it just adds a bit of delay to users receiving their emails. Thankfully it seems to be gradually getting better, but gone are the days of simply pointing your apps at sendmail letting them go.
What's interesting is setting up a handler for DMARC aggregated feedback reports. I determined that it's not worth doing yourself, even Microsoft do not handle their own DMARC reports.
I ended up outsourcing DMARC report processing, if I wanted to do it in-house I'd have to setup and maintain an Elastic stack[1] which seems like huge overkill.
This is valid not just for Mattermost but many other apps that support sending e-mails. And that's why it's important that instead of giving up and outsourcing it, we all try to set it up and have first-hand experience in challenges we have to face in the process. Because if everybody just uses Mailgun & SES, in a decade or two it won't be be possible to set up your e-mail server anymore (of course you will be able to, but your e-mails won't reach most customers).