If anyone over her is from the UK and has access to xDSL then have a look at
my ISP AA (Andrews & Arnold Ltd) Home::1 300GB / no upload restriction …
https://www.aa.net.uk/broadband/home1/
Setting up reverse DNS (domain not hosted with them) was not a problem
https://support.aa.net.uk/Reverse_DNS
No port blocking, no filtering 
Maybe we can have a list of ISP that are friendly in running your MAIB/mail server from home?
I have Dell OptiPlex 9020 Micro that I picked up on eBay
with 120GB M.2 (22x80mm) SSD and 2TB internal 2.5” inside.
2TB HDD was set up on install as /home partition 
And quote from their website:
“By default a fixed IPv4 ‘WAN’ address and a /64 IPv6 block will be assigned. If you’d like a block of IPv4 addresses then please contact us and we’ll assign a /30 or a /29 upon request. Further IPv6 blocks can be assigned too, as needed.”
I am using RPi for less intensive projets RasPBX, NTP, ADS-B, Log server, TVHeadEnd+PiTV Hat, but also using Dell OptiPlex Micro for TVHeadEnd with 5 USB DVB-T/T2 and one DVB-S2X (my 20M upload is not enught of the 4K streaming outside LAN) and extra HDD attached and works great and now MIAB as well and one more Micro in reserve for spare project (I can not help my self not picking up a good deal on eBay …)