Hello there, @Electrogas. I'm here to share why you can't attach a .msg file to your estimates in QuickBooks Online (QBO).

QBO has certain limitations on the file formats used within the program. This is done to safeguard the data's integrity and prevent malicious file uploads that could potentially harm your account. I suggest using only these formats or converting your files to any of them before attaching them. The supported attachment file types include PDF, JPEG, PNG, DOC, XLSX, CSV, TIFF, GIF, and XML.

I'm adding this article for detailed information: Add or delete attachments in QuickBooks Online.

Additionally, you can check this article on how to personalize your sales forms: Customize invoices, estimates, and sales receipts in QuickBooks Online.

Please let me know if you have further questions about file formats in QBO. I'm always happy to help. Have a great day!