Hi Tangoball76,

Thank you for posting your question in the Microsoft Q&A forum.

Based on the tag you selected, I understand you’re seeing this behavior in Outlook using a work (Exchange/Microsoft 365) account. Please correct me if I’m mistaken.

Since this is occurring with your company account, have you already informed your IT administrator?

To help us quickly narrow down the issue, could you please share the following details:

• Which Outlook experience are you using? Classic Outlook for Windows or the new Outlook for Windows.
• When did the problem start? Did it begin after an update or any recent changes (e.g., add‑ins, account settings, OS updates)?
• Do you see any specific error code or message in the Send/Receive progress window?
• Does this happen with all emails, or only with messages that have large attachments?

In the meantime, please try these recommended steps:

• Sign out of Outlook and sign back in, then test sending again.
• If you’re using Classic Outlook, consider creating a new Outlook profile and testing with it: https://support.microsoft.com/en-us/office/create-an-outlook-profile-f544c1ba-3352-4b3b-be0b-8d42a540459d

Once we have the details above, we can recommend the next best steps.

Note: Follow the steps in our documentation to enable email notifications if you want to receive the relevant email notification for this thread.

Hello,

Thanks for your answer.

This only happens if you have a second (shared) mailbox in your outlook profile.

We use Outlook 2024 LTC (Classic) and Exchange SE.

I am the Exchange-Administrator.

If I make a new profile, the Error is still there.

We tasted Outlook 2024 on Windows 11 and Windows Server 2025. Same Problem.

No Error-Messages in Eventlog.

With Cache-Modus, everything work fine. Only in Online-Modus this happens. In Owa there is no Outbox directory. With Outlook 2013 it works