Hi,
Sorry for the misunderstanding.
The type of the emails from the two modules are different.
The email from the Marketing module is called Marketing email, which is a powerful tool for your marketing campaigns.
And the email from Customer Service module is called Activity email.
Activity emails get delivered to just one or a few people, whereas Marketing emails get delivered to many thousands or more at a certain frequency.
So the Activity emails cannot be selected in Marketing module due to the differences in type and purpose of use.
If this helped you, I'd appreciate it if you'd mark this as a Verified Answer, which may in turn help others as well.
Best Regards,
Nya