When Outlook asks for Account ID, I'm never really clear about whether it means the full email addy, or just the word before the "@".
That's because it is up to each individual ISP to decide what the "account ID" is.
Some ISPs use your full email address for your account ID. Some just use the part of your email address before the @. Some ISPs don't use either one; instead, they use your first name, or some other user name.
The account ID can be anything that particular ISP wants it to be. There aren't any consistent rules. You always have to find out your account ID from the particular service provider you are using.