Most ISPs won't let you run a server unfortunately so even if they do offer static IPs they may still not let you use it to run a server. That and lot of RBLs block entire residential ISP ranges by default as it is assumed that if mail originates from a house/internet user directly it's probably spam. Kinda hate this blanket stuff myself, as I'd love to be able to run my web server from home.
Your best bet is to lease a dedicated server from a provider like OVH, then it will work. Though if the previous owner of that IP used it for spam you will then have to unlist yourself from all the RBLs, and some RBLs have a 3 strike rule and won't remove an IP more than so many times. Running a mail server kind of sucks in that aspect. But it's also nice to have full control over your mailboxes, how many you create, etc.