Are they outgoing emails from you to the client or incoming from the client to you?
The easiest way to keep track of outgoing emails is to compose the email in a filemaker text field and send it out from Filemaker. That approach allows you have additional fields in the same record as your email text, to use for linking it to other related tables.
Incoming email will be more of a challenge. I suppose you could insert references to PDFs of your emails into container fields, or you could copy and paste the email body into a text field, but maybe another forum contributor has a better idea.