Open an Outlook .msg file
.msg is Outlook's format and nobody else's: on a Mac, on a phone or with Gmail it stays a file that will not open. Here you read it straight away, download its attachments and, if you need to, save it as .eml, which anything can open. All inside your browser: the file is never uploaded anywhere.
Why a .msg opens almost nowhere
Because it is not an email: it is an Office archive, the same container as the old .doc files, with a separate piece inside for every part (the subject, the sender, the text, each attachment). A normal email (.eml) is a text file every mail program understands; a .msg is understood by Outlook. That is why the file «from a colleague» opens for them and not for you, and why the first thing to do is save it as .eml: from there you open it with Mail, Thunderbird, Gmail or whatever you use.
The sender's address that is not an address
In a company's internal mail Outlook does not store [email protected] but the path inside the system, which looks like /O=COMPANY/OU=EXCHANGE ADMINISTRATIVE GROUP/CN=RECIPIENTS/CN=MARIOROSSI: that is why so many viewers show that unreadable line instead of the sender. The real address, when it is there, is stored somewhere else in the file, and that is where this page looks. If it really is not there we say so, instead of pretending that code is an address you can reply to.
The text hiding inside the RTF
Outlook lets you write mail in «rich text format», and then the text is neither plain nor HTML: it is compressed with a method used only here, and inside there is often HTML in disguise. Many tools stop right there and show an empty page. This one decompresses it and, if the HTML is inside, puts it back together; if it really is RTF, it pulls out the words and tells you the formatting is gone.
Attachments, and the images that are not loaded
Attachments are downloaded one by one, with their name and size, and come out exactly as they were. If there is another message attached inside (common with forwards) you can open it in place and come back. Images stored inside the file are shown; images that live on the internet are not loaded, and we tell you how many: loading them would tell the sender that you opened the mail, when and from where.
What it does not do, and where the file ends up
The file stays on your device: it is read in here and never uploaded, which for a work email with its attachments is not a detail. It does not verify digital signatures and does not open rights-protected messages (IRM): those stay unreadable here too. It does not change the original .msg. If what you have is already an .eml use that page, which is made for it.