Which SMTP provider should I use for email marketing if my sending IP has issues?

Hi! If your deliverability issues really are tied to the sending IP, the “best” SMTP provider is usually the one that lets you control and isolate reputation per client, gives you strong authentication/support tooling, and has solid feedback/monitoring—because in an agency setup, one bad sender can drag everyone down on a shared pool.

Here’s what I’d look for in an SMTP provider for higher-volume, multi-client email marketing:

1) Shared vs dedicated IPs (and how to avoid “cross-contamination”)

• Shared IP pool: Fine if the provider strictly polices senders and you’re small/new, but it’s also where you can get hurt by other customers’ behavior (and where your clients can hurt each other if you’re all mixed together).
• Dedicated IP per client (ideal when volume justifies it): Best reputation isolation, but you must warm it up and keep volume consistent. If a client sends sporadically, dedicated IP can actually be harder to keep healthy.
• Dedicated IP pool for your agency: Can work if you centrally enforce list hygiene, consistent sending, and segmentation across all clients—otherwise one client’s spikes/complaints can still harm the pool.
• A real “in-between” option: Some providers offer managed/“pro” IPs or reputation-managed pools—useful if you want isolation and guidance without doing all the reputation ops yourself.

Rule of thumb: If you can’t strictly control every client’s list quality and cadence, prioritize separation (by IPs and/or by subaccounts with distinct tracking domains/return-paths).

2) True multi-tenant controls (agency-friendly features)
You’ll want:

• Subaccounts/workspaces per client with separate credentials, billing tags, and clear permissioning
• Separate suppression lists per client (plus an optional global suppression list for your whole agency)
• The ability to set per-client sending limits, rate limits, and approval flows
• Easy separation of domains, tracking, and bounce/complaint handling per client

3) Authentication + alignment support (this is non-negotiable now)
At a minimum, your provider should make it easy to implement and verify:

• SPF + DKIM for each client sending domain
• DMARC (and help diagnosing failures)
• A dedicated/custom return-path (bounce domain) and tracking domain options so your authentication stays aligned
• Support for TLS and modern sending standards (not usually a differentiator, but you don’t want a “barebones” sender)

Also, for agencies: make sure each client sends from their own domain, not a shared agency domain, unless you have a very specific reason (shared domains can create shared reputational risk).

4) Reputation tooling + deliverability monitoring
Look for built-in (or easy) access to:

• Bounce classification (hard vs soft, block bounces, etc.) and clear remediation hints
• Complaint/feedback loop handling where available, with automatic suppression
• Block/bounce trend alerts (spikes matter more than averages)
• Webhook/event streaming for bounces, complaints, unsubscribes, deferrals—so you can automate list hygiene
• Basic deliverability dashboards: delivery rate, spam placement signals (where available), engagement trends, domain/IP reputation indicators

And regardless of provider, you’ll want to actively use mailbox-provider reputation tools (your clients’ domains/IPs) and keep an eye on blocklist indicators—your SMTP provider should at least make that easy to diagnose.

5) Sending policies, limits, and enforcement (protecting you from your own clients)
This is where a lot of agencies get burned. Pick a provider that:

• Clearly supports marketing email use cases (some are primarily transactional)
• Lets you control throttling/ramp-up and has sane defaults
• Has strong anti-abuse enforcement (it’s annoying, but it protects shared reputation)
• Doesn’t force you into a one-size-fits-all setup when clients have different volumes and cadences

6) Compliance and unsubscribe handling
Even if you’re sending “opt-in,” deliverability depends heavily on how unsubscribes and complaints are handled:

• Make sure you can implement one-click unsubscribe behavior and that unsubscribes are honored fast
• Ensure the platform supports (or doesn’t break) standard unsubscribe headers and suppression logic
• Confirm how the provider treats high complaint rates and what they expect from list hygiene

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Should an agency ever run its own SMTP servers?

Sometimes, but for most agencies doing email marketing, it’s usually not worth it unless you already have serious mail infrastructure experience and a strong reason (cost at massive scale, specialized compliance requirements, or deep customization needs).

Running your own SMTP means you own everything that affects deliverability:

• IP reputation management and warm-up for multiple IPs
• Bounce processing, complaint handling, suppression lists, and unsubscribe compliance
• Feedback loops (where they exist), postmaster tooling workflows, and troubleshooting
• Security, abuse prevention, monitoring/alerting, and 24/7 ops
• Constant tuning to avoid spam folder placement issues

In practice, agencies often do better with a managed email-sending service, then focus their energy on what actually moves inbox placement: list quality, segmentation, sending cadence, content, and consistent authentication.

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If you tell me roughly (1) your monthly volume range, (2) how many clients you send for, and (3) whether clients send consistently or in bursts, I can suggest a sensible shared vs dedicated strategy (and whether “one IP per client” is realistic for you).