@Dennislmbro:
It is likely you need to have an app-specific password set up for your Mac Mail account.
I am running the latest version of Office-365 2016 on my MS-SurfaceBook2. I have iCloud set up in my Outlook mail. I also have my Gmail account set up in Outlook.
My @mac.com email account requires a sixteen digit (plus dashes) app-specific password generated on the iCloud.com site to have my @mac.com mail function in Outlook on the MS-Surface2.
I have 2 factor sign-in switched on in my iCloud.com this may or may not affect you. I have seen earlier posts that discuss this -- the two factor sign occurs each time a new machine or a new browser tries to retrieve mail from iCloud that doesn't have that
password embedded in it. iCloud sends a text to my phone with a code to authorize a new entry into my iCloud account online.
To have the software (i.e., MS Outlook) dial in automatically, it needs the app-specific password that is created in iCloud.com as stated above. Once it is in Outlook (in theory), you won;t have to re-enter it (supposedly - but I had an issue with this two
days ago - see below).
Two days ago, my smtp.me.com outgoing server "lost" or "forgot" my password following an iCloud software update. When I attempted to send an email using my mac.com email account in Outlook, a dialog box appeared telling
me I needed to enter the password for the iCloud outbound server. I am assuming that the iCloud software update wiped the password. I entered the email specific password I created when I first got the thing set up in outlook, and voila, the email was finally
sent out from the smtp.me.com server.