Adding the "From" dialogue box for sending with Windows devices

You can add the From field when you go to send an email.
Select> New email

Select the Options tab
Under the "Show Fields" select From

Using the From drop down you can select the fax email you want to send from.

If you do not change the "From" to a fax shared mailbox it will fail to send.

In the To... line enter the ten digit fax number you are sending to followed by @efaxsendsecure.com

If you use Outlook on the Web. When you select new email, select the Options tab than the Show From box. You should now be able to type the name of the Fax mailbox you have permission to send as. If you don't see the email address in the drop down, select Other email address... and search for the correct fax mailbox (if you don't have permission to send as, you will not be allowed to send a fax).