EDIT: If you find this devastating problem in a similar position, even if it’s a somewhat easy change in the code base, it’s not worth the hassle. I’ll recommend to look else where, some suggestions are Odoo, Listmonk and Mautic they offer this out of the box (which is resonable). Seeing that this feature has been requested for years and no one in the Ghost team adds it, is just weird. I feel there is an agenda in play here. Otherwise, it seems like an awesome project, good luck.
I got really excited when I found Ghost as a good complement to my business, I got less excited that I need to do a fork and do manual changes in the source code in order to use it as needed for my business. What you provide that bulk email are impossible from your own SMTP server, is simply not true and not providing the simple option of signing into a regular SMTP client is super strange. Getting my server blacklisted is not a risk on you, it’s a risk on me.
Mailgun is also a bad option and there are better out there, I strongly believe you have some kind of affiliate thing going on there even if you won’t admit it, I’m just getting weird vibes.
I’ll give Ghost a chance and see if it would be simple to do the code changes in order to use custom SMTP,