Outbox will not send, error code 0X800CCC78 with 550 sender restricted.

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Dale Hochevar

Outlook version
Outlook 2010 32 bit
Email Account

I am not happy, I just typed a thread and the forum did not post it and upon recovery all details were gone.

I can receive e-mails and have done everything on your past threads, Microsoft Support suggestions, and your website detail but cannot get my Outbox to send messages. I have not changed anything and have had this business computer since July. It worked fine and then this happened in October, December, last week, and today. It starts working on its on sometimes the same day and sometimes a day or two later for no reason whatsoever.


I work from my home and have important quotes to get out!!!!



Forum Admin

Senior Member
Sorry about that. The forum *should* remember at least some of the text - it autosaves every 20 seconds or so. There is a Restore button in the lower left of the reply window - but its sometimes easy to miss.

Do messages send if you don't view the Outbox?

Do they send if you use Safe mode?

If yes to either of those questions, an add-in is causing your problems. ICloud, OneNote, and antivirus scanners are common culprits.

Do you get any error messages?


Dale Hochevar

Outlook version
Outlook 2010 32 bit
Email Account
I only viewed the Outbox because it did not send and I saw the send/error flag at the bottom. I tried it in the safe mode and no luck. The error specifcally reads: 0x800CCC78 verify e-mail address, server responded: 550 sender is restricted.

Diane Poremsky

Senior Member
Outlook version
Outlook 2016 32 bit
Email Account
Office 365 Exchange
550 sender is restricted means the account needs to authenticate with the mail server - it can also mean the email host has blocked your account from some reason. Have you contacted them?

Go to File, Account Settings and double click on the account name. Click More settings and check the settings on the Outgoing server tab and also on the Advanced tab.

You'll need to verify with your email host what the correct settings are, but generally speaking, you want to use the option to use the same password as incoming mail on the outgoing server tab. You may need to use port 587 and SSL on the Advanced tab.

A screenshot of the dialog is at How to Send Email When Traveling or Using Public Wi-Fi - Slipstick Systems
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