Email-to-fax services act as a converter between two incompatible systems: your email client and the recipient's fax machine. Here's the full pipeline, from the moment you hit send to when the document prints on the other end.
mail
You Compose Email
Attach doc, use special To address
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Service Receives
Parses fax number from address
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File Converted
Doc → TIFF/PDF fax format
phone_forwarded
Number Dialed
Service dials recipient over phone network
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Fax Transmits
Document sent page by page
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Confirmation Email
Success or failure emailed back to you
Understanding this process explains two things: why there's no “free” way to fax from email (someone has to run fax servers and phone lines), and why things break — any link in that six-step chain can fail.