What Is a Shared IP?
TL;DR
A shared IP is a sending IP address used by more than one sender, which is the default on almost every hosted mail platform. Its reputation is the sum of what everyone on it does, so a small sender inherits both the steady volume history that makes the address trusted and the risk that another tenant damages it.
When is a shared IP the right choice?
At the volumes most outbound teams actually send. IP reputation is built from consistent traffic, and a sender who cannot supply that consistently is better off inside a pool that can, because an under-used address of your own looks unfamiliar to receivers every time you use it.
The threshold is about sustained volume rather than ambition. Moving to a dedicated address before you can keep it busy trades a shared history you did not build for no history at all, which is a worse position than the one you left.
What can go wrong on a shared IP?
Two things, and neither is your doing. A neighbour sending badly can get the address listed on a blocklist or throttled at a large provider, and your mail is deferred or refused alongside theirs. The symptom is a sudden change in delivery with no change on your side.
What you can do is narrow. Confirm the problem is the address rather than your domain, since domain reputation travels with you and would show the same symptoms everywhere, and then escalate to the provider, whose job it is to police the pool and move offenders off it.
Frequently asked questions
Not directly, because providers do not publish tenant lists. What you can see is the behaviour of the address: its listing status, and at Microsoft the IP-level data in Smart Network Data Services, which reports on the address rather than on you.