SMTP relay is the process of sending email through a third-party mail server — rather than your own domain's mail server or a hosted platform like Google Workspace. SMTP relay services like SendGrid, Mailgun, Amazon SES, and Postmark are widely used for transactional email (password resets, invoices, notifications) and marketing email. For cold email specifically, SMTP relay presents significant deliverability challenges that make it a poor choice for most outbound prospecting campaigns.
Why SMTP Relay Is Problematic for Cold Email
Most SMTP relay services operate shared IP pools — your emails share sending infrastructure with thousands of other senders. If any of those senders generate spam complaints or get blacklisted, your reputation is affected. Services like SendGrid and Mailgun are also heavily associated with mass email sending in the inbox provider models, and emails routed through their infrastructure are subjected to heightened scrutiny by Gmail and Outlook spam filters. Dedicated IP addresses on relay services resolve the shared reputation problem but require volume commitments that most cold email campaigns cannot meet.
When SMTP Relay Makes Sense
SMTP relay makes sense for transactional email (order confirmations, account notifications) where you need reliable delivery of high volumes of expected email. For cold outbound prospecting — where the recipient has no prior relationship with the sender — Google Workspace or Microsoft 365 mailboxes on dedicated domains consistently outperform SMTP relay on inbox placement.
The Right Infrastructure for Cold Email
Professional cold email uses Google Workspace or Microsoft 365 accounts on dedicated sending domains, not SMTP relay services. This is the standard that Omni configures for all client campaigns. The full infrastructure setup is described in the cold outbound system overview.