Prevent multiple calls to callback

[SergeS]

Description

Ensure that callback is called only once at the end

Motivation and Context

When emails are from different domains, ensure that callback is called after all of the emails are sent

How Has This Been Tested?

Just try send email to two different domains (Expected - callback is called when all emails are sent)

Types of changes

Bug fix (Do not return response from sending - is it really necessary ?)

Checklist:

• [X] My code follows the code style of this project.
• [X] I have read the CONTRIBUTING document.