In order to send an email and have it create a Fax, you'll need to whitelist the email address in the Skyetel portal. You can do that in the Phone Number Features options:
From here, you can add up to 25 email per Skyetel phone number:
Once this is done, simply send an email to [destination fax number]@fax.smtpengine.io with a PDF of the fax you wish to send. For example - if you want to send a fax to 3609865200, you would email a PDF to firstname.lastname@example.org from an email address that has been previously whitelisted in SkyeFax. The source phone number will be the number you associated the end user email with.
Note: Your email and attachment sent to [destination fax number]@fax.smtpengine.io must be less than 5MB in total size. If this size limit is exceeded, your request will fail to process without an error.