LetterAgent
Mail physical letters by just asking your AI assistant.
Yes. The sender number shown on a fax can be faked, just like caller ID on a voice call. The header line printed at the top of every page is set by the sender's own machine or software, so it is a claim, not proof, of who sent it.
Two layers, and both are trusting. First, the caller ID on the phone call that carries the fax can be spoofed with the same tools used to fake caller ID on robocalls: the number your machine displays when the fax comes in is whatever the caller chose to send. Second, and harder to notice, the header line printed across the top of each received page is filled in by the sender's own equipment. On a traditional machine you type that number into the setup menu yourself; on an online fax service it comes from the account settings. Nothing in the fax protocol verifies that the number in the header matches the number that actually placed the call. So a fax can arrive showing a hospital's number in the header while originating from somewhere else entirely.
Email grew up with this problem and built defenses. Modern email checks whether the sending server is authorized for the domain it claims (the checks behind SPF and DKIM), and receiving systems flag or reject mail that fails them. Fax never got an equivalent. The fax standards date from an era when the phone network itself was the identity check: if a call came from a given line, that was taken as proof. Voice-over-IP and online fax gateways broke that assumption, because a fax can now be injected from anywhere with an internet connection while claiming any number in its header. The result is that a fax's sender line deserves less trust than an email's, which surprises people who think of fax as the more secure medium.
| Situation | What to do |
|---|---|
| Unexpected fax with instructions (wire money, change bank details) | Do not act on the fax alone. Call the supposed sender at a number you already have, not one printed on the fax, and confirm. |
| Fax claims to be from your bank, insurer, or doctor | Check whether the content matches what you expect. A real office will confirm by phone without hesitation. |
| Header number differs from the caller ID your machine showed | Treat that as a red flag. The two should normally agree. |
| Routine expected fax (a form you asked for) | Low risk. If you were waiting for it and the contents match, it is almost certainly genuine. |
The callback rule is the whole defense: any fax that asks you to do something consequential gets verified through a separate channel before you act. Scammers count on the fax feeling official enough that nobody checks.
It cuts both ways. When you send a fax with your number in the header, the recipient has no technical guarantee it came from you, which is one reason sensitive recipients call back to confirm important faxes. And if someone spoofs your number on a fax you never sent, you may hear about it from confused recipients. Keep your own fax number's header accurate, keep records of what you send, and you will be able to sort out any confusion quickly.
LetterAgent's fax feature is currently in testing, so faxes are quoted and handled with care while activation completes. Ready to fax it? Fax with LetterAgent: describe it to your AI assistant, approve the exact quoted price, and it is sent.
LetterAgent is a working name. Prices include print, postage, and our service fee. US prices in USD, Canadian prices in CAD (Canadian prices exclude applicable sales tax, added at checkout). Currently in private testing.
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