[mechanical_fish]

Hosting your own email works great until you try to actually send any. [1] But if you send enough, you eventually have to deal with the spam infrastructure. Customers will complain that critical transactional emails ended up in user spam folders. One person too many will accidentally mark one of your mails as spam. Or you'll relaunch some cloud infrastructure and suddenly your IP is one that is on all the blacklists because of some earlier history that you could never have known. Or one of your customers will innocently assume that you are Mailchimp and send a whole bunch of marketing email through your server.

The symptom of any such problem is that email starts falling into the garbage, perhaps silently. But if you are watching your stats like a hawk (a costly and very boring hobby ) - or, much more likely, if your customers start screaming about the very-real high costs of lost email - you will figure that out within a few days, so it might not take more than another day to figure out how to get yourself off the blacklist (a manual process) and how to apologize to customers for the lost email (a manual process). But, if you value your time at in any way, your first encounter with that problem will be your last, because email services are cheap and easy. I have yet to pay for mine; in small quantities Mailchimp, Mailgun et al are free. Indeed, given that low cost, why wait to get burned before you stop playing with fire, even if the fire looks cute and innocent and warm right now?

[1] Actually, I'm lying: A much more serious problem which occurs at any volume is that email is another piece of the security perimeter. Even if mail servers had almost no configuration and a history of bug-free operation - which they do not, I am joking, and the joke is not very funny - one of the basic rules of security is to put as much distance as possible between your data and services which connect to outside machines. Mailgun is free in small quantities and runs outside your entire datacenter. The math kind of does itself.

[logn]

I was referring to just normal, non-automated emails above. An alternative to say, Google Apps, where I can fit an entire email server on a VPS with less than 400 MB memory, vs $10/user with Google.

I agree with you for transactional email.

For any DIY project there's a limit where you really should just pay someone. But for instance I run wrote my own Stripe-based invoice generation, subscription management, and dunning vs paying for a service. It works for me for now. If one day my little app breaks down if/when I have thousands of customers then I'll outsource it.

I run my own private Git repo vs paying $7 to github. Etc.

Edit: to add, I am selling software I wrote basically from scratch (and there's no way around that). Like a comment way up above, my biggest risk is in that stack. I suppose with the uber-paranoid mindset I should have outsourced my own development work. At the end of the day, someone like me is responsible for something important. It's not life or death with my software so at least I got that.