Developer API FAQ

Q1. Is developer faxing really free?

Yes, as long as you are using InterFAX as part of an effort to build or debug an application, you can use the service for free to send faxes to your designated fax number. Once your application goes live, you need to start faxing through a live, paid account (but you can keep using your developer account for any debugging or future enhancements). The rule is simple: if you’re testing, your welcome to use a free developer account.

Q2. Why is there a prepaid card in my account when I never bought one? Am I being charged?

No, you are not being charged. When you register for a developer account, your account behaves exactly the same as a regular user account would, with two exceptions: a.) your faxes are free, and b.) you can only fax to one designated fax number. This was designed so that you can experience the InterFAX service exactly as your client will, especially at crucial points such as when a prepaid package is about to expire. Towards this goal, we feed a $10 prepaid package into your account when you register, so that you can view the depletion of your outstanding credit as you send faxes.

Q3. What do I do when my prepaid card runs out?

If your prepaid card runs out and you start receiving ‘Out of quota (205000)’ error messages – simply contact us and we will insert another prepaid card into your account.

Q4. I’ve sent an email-to-fax but I don’t see the fax in my outbound queue. What’s happening?

The solution to this comes in three steps:

  1. Make sure that your “From:” address is the one you registered with.
  2. Make sure that the “From:” address in your email is in your list of “Allowed Senders” (Login -> Preferences -> Outgoing -> Sending Permissions).
  3. Verify your “From:” address (for example, by sending yourself an email from the same workstation from which you’re trying to fax).

Verify that you have appended to the properly formatted fax number in the email’s “To:” field.

Q5. Why can’t I send an email to fax from PHP, when I succeeded faxing from my regular email client?

When sending an email to fax from PHP it is not enough to set the “From:” address in the mail() command. You must also set the sendmail_from address in the [mail function]section of PHP.INI to be the same as an allowed sender in your Interfax account. (For a full tutorial on emailing from PHP see

If your server is in a hosted environment and you have no access to PHP.INI, try using ini_set() to change the sendmail_from value.

If all else fails, contact us and we will try to determine what the sendmail_from value of your server is, then enable faxing from that address.

Q6. Why is my email getting rejected/ why is the Web Service giving me error code -111?

Because you are attempting to fax to a number that is not the designated fax number in your developer account.

Q7. Will my clients only be able to fax to one number?!

No, regular (paid) accounts don’t have this limitation.

Q8. How do I fax to any number from my developer account?

By turning it into a regular (paid) account. Simply login to your account and select Purchase -> Prepaid Cards and select the size prepaid package you wish to buy. Note: once you place a purchase from within your developer account, free faxing to the designated fax number is no longer available, but you can always open an additional developer account for this.

Q9. How do I change my designated fax number?

To avoid abuse of the free developer account, this number cannot be changed by the account holder. Please contact developer support with the number you wish to change to.

Q10.I’m a student. May I use the free developer program as part of a project (even though I don’t realistically intend to buy from you)?

Yes, feel free to use the service as part of your project. Remember to give us credit, if you get the opportunity.

Q11.I have an application that will send a very large number of faxes. I would like to test InterFAX under load. Can you give me sufficient credits to stress-test it?

No, InterFAX already functions under load, sending tens of millions of faxes per year. The system is planned and scaled to serve our clients’ needs. If you wish to run stress tests, you’re welcome to buy faxing credits to do so.