What’s the difference between native SMTP and third-party relay?

The sending infrastructure choice affects how your email appears to recipients and mailbox providers.

Native SMTP:

Connects directly to your mailbox provider (Gmail, Outlook, Google Workspace)

Messages appear to come from your actual email account

Uses the provider's sending infrastructure

Subject to provider's sending limits and policies

Standard approach for cold email tools

Third-party relay:

Routes email through a sending service (SendGrid, Mailgun, Amazon SES)

Uses the relay's IP infrastructure

Higher volume capabilities

Typically prohibited for cold email by relay terms of service

Why cold email uses native SMTP:

Messages appear as individual sends, not bulk

Leverages personal mailbox reputation

Enables true threading and reply detection

Mimics human sending behavior

Tradeoffs:

Native SMTP has lower volume limits

Relay offers better deliverability infrastructure (for legitimate use)

Cold email requirements don't fit relay terms of service

Native approach is appropriate for outreach use case