You should not use Microsoft Outlook for any email marketing! Particularly if you are using HTML messages.
Microsoft Outlook, because it uses Word as its editor, will insert all kinds of nonsense code into your HTML. It will not render correctly in many ISPs.
Now, if you are sending plaintext, I suppose Outlook would suffice - only if your recipient list is very small. ISPs will begin blocking your IP Address, and managing unsubscribes is much more difficult (as a manual process).
Seriously consider an email marketing tool (like I said, Fathom
SEO is great!) for any kind of email marketing.