I am finding that I email people snd get responses on other email addresses I don't want them on. If I am responding to an email in one account shouldn't it default to that account? I know if I am started a new email I need to select.
Apple mail can be very uncooperative. Always examine the from field when you send from an Apple computer or device. I argued this with a Genius in the Apple Store some months back and found that my concerns fell on deaf ears. At the time, I used gmail to "aggregate" half a dozen email addresses. I found that when I replied, the "from" address was not the identity that received the email, it was the gmail account I happened to be using to check email. I have had to go through some gymnastics to scrub my gmail address (which I never intended to give out) from all my correspondance but the root cause of the trouble I'm having is Apple's stubborn insistence that I can't necessarily "reply as" the identity I choose to use.
It's less severe on OSX than it is on iOS but it still takes some extra steps to make sure you are replying as who you think you are replying as.
In mail.app, be sure to pick a default email to use when you send email. And yes if you set mail.app up to check 3 or 4 accounts, even if you have a default outbound email account, mail.app should default to replying from the account you are using when you reply. But even knowing mail.app defaults to replying from the "correct" account and after setting a default outgoing email, you have to pause and look at that from field before you hit send
every time.
By contrast, google apps allows you to send and receive from your own domain using gmail. I've started porting all my email to google apps as I grow tired of the inflexibility of iCloud and I demand absolute control over my identity when I send email.