This is one of the two genuine reasons to hold a dedicated IP.
THE PROBLEM ON A SHARED IP
Outbound mail from the server is associated with the shared address. Receiving providers judge that address by the aggregate behaviour of everyone using it.
If another account is compromised and sends spam, the shared address can be listed on blacklists, and your legitimate mail starts being filtered until it is resolved.
WHAT A DEDICATED IP CHANGES
Your sending reputation becomes entirely your own. Nobody else's behaviour affects it.
THE OTHER SIDE
It also means your own mistakes affect only you, with nothing to dilute them. A new dedicated IP starts with no reputation at all, which receiving providers treat cautiously until a sending history is established.
WHO BENEFITS
Businesses sending a steady, moderate volume of legitimate transactional mail who want reputation they control.
WHO DOES NOT
Occasional senders, for whom the shared reputation is generally fine and a new unwarmed address may perform worse. Bulk senders, who should use a dedicated sending provider rather than hosting at all.
