| File Extension: | LETTER |
| File Type Description: | Undeliverable Mail File |
| File Type Developer: | Proofpoint, Inc. |
| Primary Associated Software: | Sendmail |
| Primary Software Developer: | Proofpoint, Inc. |
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Sendmail is the primary software program that utilizes the Undeliverable Mail File file, originally developed by Proofpoint, Inc.. Commonly, Undeliverable Mail File files are found on user computers from United States, and on PCs running the Windows 10 operating system. A high percentage of these users browse the web using Google Chrome.
 | Sendmail by Proofpoint, Inc. |
| Windows 10 | Windows 11 | Windows 6 | Windows 8.1 |
 | Linux |
| Extension | File Type Developer | File Category | File Type Description |
|---|---|---|---|
| .MC | Sendmail, Inc. | Data Files | Sendmail M4 Configuration |
| .CF | Proofpoint, Inc. | Settings Files | Sendmail Configuration File |
| .D11 | NorthStar Solutions | Data Files | Macro Mania Data File |
| .LWR | Atari Corporation | Data Files | RollerCoaster Tycoon Laser Writing File |
| .LKO | Microsoft Corporation | Data Files | Outlook Express Linked Object |
| .URF | Matinsoft Inc. | Settings Files | AppFace Skin File |
| .EFT | Cay Horstmann | Font Files | ChiWriter High Resolution Screen Font |
| .TDH | Borland | Data Files | Turbo Debugger Help File |
| .ZPZ | Check Point Software Technologies Ltd. | Data Files | ZoneAlarm Mailsafe Renamed Data |
| .FLP | Image-Line | Audio Files | FL Studio Project File |
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