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