Microsoft have not exactly been helpful recently and they have altered several tihngs. All email addresses in the Microsoft Account are now regarded by Microsoft as 'alias' accounts. Whether they are hotmail.com, live.com, msn.com or outlook.com or any other email address that uses the outlook office365 servers. They need to use same 'password' as the Microsoft Account. One of the email addresses will be set up as 'primary'. You can chose whatever is set up 'primary', but whatever you set up can only be used as the smtp server for all email addresses/mail accounts in third party email clients. You said " I wonder if there is a conflict when I have both a school email account and a personal outlook account in Thunderbird. " There is no conflict with incoming mail, but when it comes to 'sending', you can only send via whatever is set up as primary account. So if you can send via your 'school' account then it's likely that school account is set up as the primary account. So, your outlook account may only be able to send using the smtp used by the school email addess. Unless you change the 'primary' account to outlook, then both accounts would use the outlook as smtp for sending.

Thanks for your reply. Yes, I have no trouble receiving emails in both my school email and the outlook email. I do have trouble sending out the outlook emails.

After I deleted the outlook email's SMTP server, set the school SMTP server as the default (which is smtp.office365.com), and tried to send an Outlook email, I received an error message: Sending of the message failed. An error occurred while sending mail. The mail server responded: SendAsDenied; xxxx@fit.edu not allowed to send as yyyy@outlook.com

Note xxxx and yyyy are pseudo names. Please let me know if you have any suggestions. Thanks.

I should have added that I used the Outlook SMTP server as the default. However, I still could not send emails from my Outlook email.

In Microsoft account accessed via a browser which email address is set up as 'primary'?

In my google browser, I don't sign into my MS account. When I open MS account login page, it gives me two options to sign in: using school or outlook account.

Usually I just open two web-based Outlook tabs simultaneously to read separately my school email and outlook email. There is no problem in receiving and sending emails from either email account. I am not sure when I read/send email in google browser, which is the primary MS account.