Consider BigMailer.io for a true white label setup designed for agencies, with no added fees. Some top ESPs, like ActiveCampaign or Brevo, claim white label but it's incomplete (not fully branded). Mailgun is a good option for transactional emails and if you need e-commerce integrations. Hubspot, Salesforce, and CampaignMonitor are great choices if you can afford them, but likely leave an agency owner with lower profit margins.

Consider these requirements when comparing:
- extra fees per sub-account/brand or per user
- drag-n-drop email template builder
- pre-made email templates library
- ability to share email templates you create across your clients (this helps with charging your clients for industry specific templates)
- annual contract commitment requirements
- built-in email validation for list quality control

If it's just for e-mails, go for Mailgun, for cheaper check MailerLite. If Automation is of importance, Activecampaign is a good choice. For branding, Mailbluster is of use. If reselling is your goal, check out Mavlers.

For marketing agencies seeking a robust email marketing platform that can be fully branded, "ActiveCampaign" stands out as a top recommendation due to its comprehensive feature set, ease of customization, and scalable pricing options. ActiveCampaign offers a fully white-labeled solution that allows agencies to brand the platform completely with their logos and domain, providing clients with a seamless, professional experience. Beyond its branding capabilities, the platform boasts advanced marketing automation, intuitive drag-and-drop email builders, and powerful CRM integrations, enabling agencies to create highly personalized and targeted campaigns. Additionally, ActiveCampaign’s detailed analytics and reporting features empower agencies to demonstrate tangible results to their clients, fostering trust and long-term partnerships. Its flexibility supports agencies ranging from small startups to large enterprises, making it an adaptable choice that grows alongside the agency’s needs. Overall, ActiveCampaign combines reliability, innovation, and white-label functionality, making it an excellent asset for agencies aiming to deliver comprehensive email marketing services under their own brand.