It's getting pretty expensive to rent one IPv4 address per domain these days. You also don't always control every address in a block, which means there may be nothing you can do about your reputation no matter where you go.
You only need one IP per MTA not per domain. I have a "vanity" email system that I run at home. I run it for my mates too. I have around 10 domains inbound. It all works fine.
SMTP and SIP are often held aloft as fucked up. My Dad's home telephony runs off a RPi and a Yealink DECT station and a dynamic DNS.
The modern internet might look a bit fucked up if you only look at the X/Facebook/webby wankery stuff but the real internet is functioning quite happily.
