You may get this error message in Outlook when sending email to a particular user ( s ) due to Message Size Restriction parameters in Mailbox properties.
We won’t be able to deliver this message to John Doe because it is too large.
Usually Message Size Restriction is turned off, meaning within Exchange organization users are able to send bigger sized attachments. To increase the Message Size Restriction or to disable it, change it in the mailbox properties in Exchange console.
- Open the Exchange Console
- Search for the mailbox in Recipients –> Mailbox section
- Double click or right click the mailbox & select Properties.
- Select Mailbox features and click on View details under Message Size Restrictions.
- Uncheck boxes to disable the size restriction or enter an message size value in KBs.
- Click OKs to close and save the setting.