I agree, it's not necessarily a bad idea. I meant that most people (even most hackers) probably won't do it. I think the biggest issue is reliability, since if your SMTP server is down emails sent to you are rejected.
Meh. I had it running on a line at my home for years. It's been on Linode for the last 2.5 with near zero downtime (Linode had one outage in Texas, I've rebooted a few times, and did a distro upgrade once). Looking at moving it to AWS at the moment. Really, it's easy.
Most SMTP servers trying to send to you will try for many days before giving up. A typical configuration for a sender may get give a warning after the first 24 hours and then a bounce after another 48 hours, so normally you'll need to get your server back up and running within three days before you'll actually lose mail.
This is a bit of a misunderstanding of how SMTP works.
If you run and maintain your own SMTP server and it goes down, you won't be able to send email.
Everyone else will still be able to send email to you, and it will be delivered to you, and you'll be able to read it.
Now, if you break Postfix or something on your server and start bouncing emails then you can be in trouble, or if you mess up the DNS somehow.
But generally running your own mail server is a set it up and leave it alone type of affair. Any junior level hacker can cobble one together with guides online and have it up and running with no problems in a few hours.
Senders will keep mail for you in the queue only for a limited time, after a few days (IIRC) your mail will bounce. So if your host or ISP unfairly takes down your SMTP server and you have to go to court with them, you WILL lose mail.
Generally, running an SMTP server is quite a responsibility: not losing mail, not being exploited to send spam. What matters is not so much that you can set up in a few hours, but whether you want to take on that responsibility.
What? No. You just get another host somewhere (free amazon micro EC2, etc...) and point your DNS to that via your registrar. The only thing that can prevent mail service with longer than ~24 hours latency is a seizure of the domain.
No, the misunderstanding is yours. Postfix IS an SMTP server. SMTP servers connect to other SMTP servers to deliver mail. Your mail comes into an SMTP server just as it goes out an SMTP server.
ocelotpotpie meant that SMTP servers are designed to reliably queue mail and retransmit. If your mail server goes down, you don't lose mail. The senders will just try again later. Downtime on an SMTP server impacts delivery latency, not reliability.