You'd need to explain this in more detail. What are you trying to achieve in the first place?

In my main email account, I'm receiving email from my secondary email account. I do not wish to have email from this secondary account appear in my main email account and I assume these two accounts were synced and I would like to keep them separated

How do you define 'main' and 'secondary' accounts? Are these different accounts? Have you set up forwarding from one to the other account? Or is 'secondary' synonymous with an additional identity of the first account?

My main account is the one I use for everyday messages. The secondary account is one I use for business, social, and military messages. I don't recall ever setting up forwarding from one to the other unless I did it by accident.

re :In my main email account, I'm receiving email from my secondary email account.

If forwarding was set up then it might have been done in the secondary webmail account accessed via a browser. Worth checking it out.

re: check identities Right click on mail account name in Folder Pane and select 'Settings' click on 'Manage Identities' button There should be one listed for each mail account and it should be only the same email address as the actual account email address.

In same area at the bottom what outgoing server is selected? Check sending outgoing server smtp for mail accounts: If you compose using Account A as the FROM, but actually send using Account B smtp server then recipients may see email coming from Account B and therefore send or reply to that email B address.