Mac Mail sending from the wrong email address

In the mail app you can specify which of your email accounts is the default account to be used for composing and sending new messages. However, that does not override the fact that when replying to an email, the reply defaults to the email account that was used to receive the message.

 

It's your "Outgoing Mail" settings

I was having this problem as well, and ran into a lot of frustration trying to find an answer that actually solved my problem and that I could understand. I have three accounts connected to my apple mail app. Then I finally figured it out, and since then, it has worked correctly. Here's what you do:

In Mail, go to your Preferences. Click on one of your accounts on the left side. Where it says "Outgoing Mail Server (SMTP)" Click the dropdown menu and choose "Edit smtp Server List". When you get there, delete any and all smtp servers that you have saved. Click the + to add a new one. You'll add one server for each account you're using.

In "Account Information":
Description: anything you want. something that tells you which account you're using it for.
Server: usually smtp.gmail.com (slightly different if your account is not gmail)
TLS Certificate: None

Then go to "Advanced":
At "authentication", click "password"
Enter the username and password of the account you want for this outgoing server.

Repeat this for as many accounts as you have. Then go back to your main page with the accounts. On each account, go to the Outgoing Mail Server dropdown list and choose the server that you have created for that specific account.

Complicated, but this solved it for me. I hope it works for you!