What is the difference between a dedicated IP and a shared IP, and when should you use each?

Shared IP: Multiple senders share the same sending IP address. Reputation is influenced by all senders on that IP. Marketing Cloud's shared IP pools are managed by Salesforce and are pre-warmed. Suitable for lower-volume senders (typically below 50,000 emails/month) who cannot maintain sufficient volume to keep a dedicated IP warm.
Dedicated IP: A single IP address reserved exclusively for your organisation's sends. Your reputation is entirely determined by your own sending practices. Requires deliberate warming from cold. Typically recommended for senders above 50,000–100,000 emails/month who send consistently (at least weekly). Multiple dedicated IPs can be grouped into an IP pool for load distribution.
Key consideration: If you stop sending on a dedicated IP for more than 30 days, the reputation fades and the IP effectively goes cold again, requiring re-warming.