When to use custom SMTP

Use custom SMTP in EonMailer when the client’s mail path already exists and won’t move to a REST API provider soon.

Good fits

• Microsoft 365 SMTP relay approved by client IT
• Transactional host (Postfix, Exim) on client VPC
• Temporary bridge during ESP migration
• Staging environment with internal relay (non-production)

Poor fits

• High-volume marketing without dedicated IP/reputation tooling
• Providers that don’t support STARTTLS on port 587
• Shared hosting SMTP with opaque rate limits (often blocked)

Tradeoffs vs API providers

	SMTP	SES/SendGrid/Mailgun
Event webhooks	Often limited	Rich bounce/complaint events
Throughput clarity	Opaque	Documented quotas
Setup	Host + port + user/pass	API keys + DNS

Hybrid pattern

Some agencies send transactional via client SMTP and marketing via SES — EonMailer supports one provider per workspace, so split clients into workspaces or standardize on one path per brand.

Next: Connect custom SMTP.