On paper, a CRM and an email service provider (ESP) seem to speak the same language: both know customers, both know touchpoints. In practice, this is exactly where connections often get stuck.
Two definitions of "customer"
A CRM thinks in accounts and deals: a customer is an organisation with contacts attached. An ESP thinks in email addresses: a customer is an inbox that is or isn't open to communication. The moment you try to map those two one-to-one, edge cases appear — a contact switching companies, a shared email address, a lead without an account yet.
The technical connection is rarely the problem. The problem is that nobody decided up front which system wins when in doubt.
How we solve this
We start every integration project with a simple question: for each data type, which system is the source of truth? That question forces a decision before a single line of code is written, and prevents most of the trouble that follows.
Record which system leads for each field.
Synchronise identifiers, not just email addresses, so records keep recognising each other after changes.
Build a clear rule for duplicate or merged contacts.
Once those agreements exist, the rest is engineering — and engineering is the easy part.