The dedicated-versus-shared question gets more attention than it deserves, largely because it is the axis email service providers sell on. For a lot of senders it is the wrong question entirely — but it is worth understanding properly before setting it aside.
Shared IPs
Your mail leaves from a pool of addresses used by many of the provider's customers. Reputation is collective.
In favour: the pool usually has an established, positive reputation you benefit from immediately. There is nothing to warm up. Volume fluctuations do not hurt you, because the pool's aggregate traffic stays consistent even when yours does not. And it is cheaper.
Against: you inherit your neighbours' behaviour. A careless sender on the same pool can degrade your delivery through no fault of yours, and you will not know it happened — you will just see open rates drop for reasons you cannot explain.
Dedicated IPs
An IP address used only by you. Reputation is entirely your own.
In favour: complete isolation. Nobody else can damage you. At high volume you build a strong, specific reputation, and you get much clearer diagnostics because every signal about that IP is about you.
Against: it starts with no reputation at all, which is worse than starting with a good shared one. Warming takes four to eight weeks. And crucially it requires sustained volume — an IP that sends 2,000 messages one week and nothing for a month never accumulates enough signal for providers to form a confident view, and inconsistency itself reads as suspicious.
The commonly cited floor is around 50,000 messages a month. Below that, a dedicated IP frequently performs worse than the shared pool it replaced. This surprises people who bought one expecting an upgrade.
Comparing them directly
| Factor | Shared IP | Dedicated IP |
|---|---|---|
| Starting reputation | Established, borrowed | None — must be built |
| Warm-up needed | No | 4–8 weeks |
| Minimum volume | None | ~50,000/month to sustain |
| Neighbour risk | Yes | None |
| Tolerates irregular sending | Yes | Poorly |
| Diagnostic clarity | Muddied by others | Clear |
The third option
Both models assume your mail leaves from a sending platform's infrastructure. There is another arrangement: send through mailboxes you already own at established providers.
When you relay through Google Workspace or Microsoft 365 accounts, the IP reputation belongs to Google or Microsoft — infrastructure with about as strong a standing as exists. You are not renting reputation or building it from scratch. What you are building is domain reputation, attached to your authenticated sending domain, which is portable and which you control.
The constraint is per-mailbox volume: each account has a ceiling in the region of 80 to 144 messages an hour once sends are paced sensibly. You address that by connecting more mailboxes and distributing campaigns across them, rather than by pushing any single account harder.
This suits the large middle of the market — senders with real volume who do not have the sustained 50,000-a-month traffic that justifies a dedicated IP, and who would rather not depend on strangers sharing a pool.
Choosing
- Under 50,000/month, irregular sending: shared IP, or mailbox-based sending. A dedicated IP will underperform.
- Over 50,000/month, consistent: a dedicated IP is worth the warm-up.
- Outbound campaigns from domains you own: mailbox-based sending, with volume distributed across accounts.
- Regulated or high-stakes sending: dedicated, for the isolation and the diagnostics.
Whichever you pick, none of it substitutes for authentication, list quality and engagement. IP strategy is a second-order decision; those three are first-order, and no amount of infrastructure compensates for getting them wrong.