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Cold email deliverability doesn't just depend on what you send. It also depends on the infrastructure you use to send it. The same email sent from a poorly configured SMTP server versus a properly configured infrastructure will yield radically different results in terms of inbox placement.
Many teams use the SMTP server provided by their web host or primary email provider for their cold email campaigns. This is one of the most common mistakes in cold email deliverability, as it mixes the reputation of transactional emails with that of cold outreach emails.
1. Shared SMTP vs. Dedicated SMTP vs. ESP: Understanding the Differences
Your hosting provider’s or primary service provider’s SMTP server (OVH, Ionos, your ISP) is designed for occasional emails, not for high-volume cold email campaigns. The IP addresses of these servers are often shared with thousands of other customers whose behavior you have no control over. If one of them is flagged as a spammer, your reputation suffers.
A consumer-grade ESP (Mailchimp, Brevo, Sendgrid) offers infrastructure optimized for high volume, with built-in tools for monitoring cold email deliverability, shared IPs in pools segmented by reputation, and the option to obtain a dedicated IP once a certain volume is reached.
A dedicated SMTP server (VPS with Postfix, Amazon SES in direct mode) offers maximum control over configuration, but requires technical expertise and active reputation management. This is the option of choice for teams that send large volumes of email and want complete isolation of their infrastructure.
2. Recommended ESPs for cold email deliverability
SendGrid is one of the world's leading providers of transactional and marketing email services. It offers detailed deliverability metrics, a robust API, and dedicated IP options for high-volume senders. Its infrastructure is well-maintained, and its shared IPs generally enjoy a solid reputation.
Brevo (formerly Sendinblue) is a French email service provider (ESP) that offers similar features with built-in GDPR compliance. It is particularly well-suited for European teams looking for an all-in-one solution (email + SMS + CRM) with strong cold email deliverability.
Postmark specializes in transactional emails with high deliverability. Its IP addresses are strictly reserved for legitimate emails, which gives it an excellent reputation. It is particularly well-suited for confirmation emails, notification emails, and initial cold email campaigns that absolutely must reach the primary inbox.
Mailgun is developer-focused, with a flexible API and detailed logs. It offers great options for technical teams looking to integrate email sending into their automated workflows.
3. Shared IP vs. Dedicated IP: When to Switch to a Dedicated IP
A shared IP address benefits from a reputation built by all the senders who use it. This is an advantage when getting started: there’s no need for a long warm-up period, as the reputation is already established. It’s a disadvantage if careless senders share your pool and damage the collective reputation.
A dedicated IP gives you complete control: your reputation depends solely on your behavior. But it must be properly warmed up before sending large volumes of email. A cold dedicated IP has a zero reputation, which is almost as problematic for cold email deliverability as a bad reputation.
A general rule of thumb: switch to a dedicated IP when you reach 50,000 to 100,000 emails per month, and only if you already have best practices for email sending in place.
4. Essential DNS settings
Regardless of the infrastructure you choose, three DNS settings are required for optimal cold email deliverability.
SPF (Sender Policy Framework). A TXT record in your DNS that lists the servers authorized to send emails on your behalf. Without SPF, receiving servers cannot verify that the email actually came from you.
DKIM (DomainKeys Identified Mail). A cryptographic signature added to every email. The receiving server verifies the signature against a public key published in your DNS. This ensures that the email has not been tampered with in transit.
DMARC (Domain-based Message Authentication, Reporting, and Conformance). The policy that coordinates SPF and DKIM. It tells recipient servers what to do if authentication fails, and sends you reports on emails sent on your behalf.
These three protocols have been mandatory since February 2024 for bulk email campaigns sent to Gmail and Yahoo. Failure to comply directly impacts cold email deliverability, even for moderate volumes.
5. Separate the transactional infrastructure from the cold email infrastructure
This is one of the most important recommendations in email infrastructure design: never send your cold emails from the same IP address or domain as your transactional emails (order confirmations, notifications, password resets).
If your cold email campaign generates complaints or bounces, these issues should not affect the reputation of your transactional emails, which must reach the inbox at all costs. Our article on segmenting sending domains explains how to manage this separation.
Conclusion
Your SMTP infrastructure is the invisible foundation of your cold email deliverability. A reliable ESP, comprehensive DNS configurations, a clear separation between transactional emails and cold emails, and continuous monitoring: these four elements determine the performance of all your campaigns.
To help you structure your overall email marketing strategy, our guide to B2B email outreach and our article on outreach copywriting complement this technical guide.
