This error can appear when Outlook or another mail client sends a message with line wrapping that does not meet mail transport requirements.
It is commonly caused by message formatting issues, especially when plain text or copied content creates very long unbroken lines.
Try switching the message format, enabling proper word wrapping, removing problematic pasted content, or recreating the message before sending again.
If the issue only affects one workstation or user profile, test with a new message and review the application formatting settings.
