MailUp

MailUp has been serving agencies since 2001 and now handles over 25 billion emails annually from 800+ resellers and agencies worldwide. The platform includes white-label options so agencies can brand the entire interface with their own colors and domain, presenting it as their own solution to clients. Multi-client dashboards, flexible permission models, and reseller billing structures make it straightforward to manage dozens of separate accounts.

MailUp's longevity and scale demonstrate staying power in a competitive market. Agencies appreciate the mature feature set, reliable deliverability that comes from handling such volume, and the fact that you're not betting on a startup. Compared to younger platforms, MailUp's reseller ecosystem is already established, so you'll find community support, integrations, and proven workflows from thousands of other agencies. The white-label capabilities are particularly strong if branding consistency matters to your client relationships.