Insufficient Privileges to perform this operation only when sending mail with $(Current.User.Email)

Insufficient Privileges to perform this operation only when sending mail with $(Current.User.Email)

I run my current Zoho CRM with an administrator account with full rights.

When I send an individual email using a template to a Contact, it works correctly.
When I try to send an individual email using a template to a Contact but I try to change the email sender, it no longer works, I get the well known error message :

Insufficient Privileges to perform this operation. Contact your Administrator.

Does anybody know what I am doing wrong ?

Thanks,

Jérémie