Windows Advisor

POP3 Users: Rejected!

Admin wonders why user names in Exchange Server 2007 comes up with errors.

Q. Why do we get an error when some users log in to Exchange Server 2007 via POP3? We get an error that states that the user name was rejected.

A. We're all familiar with the user principal name (UPN); it includes a user's log-on name, the @ symbol, followed by the domain name. Errors can occur when users have a UPN that exceeds 40 characters, which is the default limit for POP3 set by the MaxCommandSize setting.

You can resolve this issue in one of two ways. The first method is just to increase the value of MaxCommandSize, this way:

  1. Start the Exchange Management Shell.
  2. Type "Get-POPSettings" (no quotes) to view current settings. You'll notice that the default value is 40.
  3. To increase the value to 50 (the upper limit is 1,024 bytes), type: Set-POPSettings -MaxCommandSize 50
  4. Restart POP3 for the changes to take effect.

The second method is to increase the value of MSExchPopImapCommandSize via the Adsiedit.msc snap-in., It's a bit more complicated, but it's covered at this KB article.

About the Author

Zubair Alexander, MCSE, MCT, MCSA and Microsoft MVP is the founder of SeattlePro Enterprises, an IT training and consulting business. His experience covers a wide range of spectrum: trainer, consultant, systems administrator, security architect, network engineer, author, technical editor, college instructor and public speaker. Zubair holds more than 25 technical certifications and Bachelor of Science degrees in Aeronautics & Astronautics Engineering, Mathematics and Computer Information Systems. His Web site,, is dedicated to technical resources for IT professionals. Zubair may be reached at [email protected].

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