3 things decide how much you can trust an IP.
→ Blacklist status. If it's on Spamhaus, everything downstream gets harder. This needs ongoing monitoring because IPs get listed overnight.
→ DNS setup. A missing PTR record is an instant red flag for mail servers.
→ Network type. IPs from real ISP networks score better than datacenter ranges, because that's where normal human traffic comes from.
Shared IPs are cheaper for a reason. Know what you're sharing.