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Meeting accept response mails are stuck in my outbox and Outlook displays an error message:

An unknown error has occurred in Outlook.

Mail could not be sent.

Error code -50

Has anyone else had this problem and found a solution?

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Yeah I'm troubleshooting this problem right now for a user. Was sad to see no one has given a response. What I've found so far is that error code -50 is paramErr - Error in user parameter list. And I've found a few solutions but I have yet to test them to see if they work. So attempt at your own risk!

Copied from online forums:

  • One user found this solution: Run "Remove Office", then run Repair Permissions. Install from your CD and apply all updates. Restart and then run Repair Permissions again.

  • Another cause for this error was found by [name omitted]: I had a semicolon in my outgoing server name.

  • Another user reported this fixed the issue for them: Try creating a new Entourage identity and setting up your email account. When the email address is verified with the new identity and you receive your e-mail, then try switching back to the identity that you had a problem with and check your mail. This solved the problem for me.

  • [Name Omitted], reported this solution: I found old mails in my outbox that were never delivered (for an unknown reason), but everytime I wrote a new mail, Entourage tried to send them again, leading to a -50 error everytime. Deleting these old entries in my outbox resolved the problem.

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Yes, I seem to have cracked it. I somehow changed my Date and Time on my local machine and saved it. Issue was that the mail synched during this brief change. I had 20 mails trying to synch mails from my future changed date as I had subsequently changed my local time back to the normal day. This caused much confusion on the exchange eg: last synched 30 January 2015 and my local time and date is the 20th January. I moved all my mails out of my inbox including sent items to an archive and re-synched my mail account. This seems to have fixed the error code 50 on trying to synch my mail.

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