MAIL problem: flashing draft, unable to delete

I have never knowingly used MAIL as my email program. I use Yahoo mail for my email program (Was using it in windows before I switch to Mac). The problem I have is sometimes when I'm on a website and I want to send them an email with a question, it opens to another type of email window with a large pointed arrow in the top left (is that MAIL?). I write the email, but when I try to send, I get a msg along the line that my server or such is not recognized. I don't know how to fix this so I try to delete the email but i can't. Not a problem. I go back to my homepage (my.yahoo), but still not solving that underlying problem. The real problem occurs whenever I hit MAIL at another time. For instance, if I attempt to log in to one of my financial sites, the financial site, for security or whatever, tells me to check my mail for a code to verify my password or whatever. I cannot go to mail in my.yahoo or I risk getting out of that financial site. When I hit MAIL to view the message, the original draft that I wrote manny weeks ago comes up and starts flashing (alternating with my current screen view). The only way to stop it...that I know of...is to restart. But then it happens again whenever I can't open my yahoo email to check for the email from the financial institution and open MAIL. How can I fix this/

If Yahoo mail is the mail program you're using on your Mac rather than the default Mail app, then any security code sent to your email address should show up in your Yahoo mail inbox. It sounds like Yahoo mail is not your default mail app even though you've been using it. Do this: Open the Apple Mail program on your Mac and select Preferences, click on the General Tab, and check to see what it says your default Mail program is.

Let me try and explain it more clearly. Sometimes when I am trying to log in to a financial site they will send a code to my yahoo email address so they can verify it is me. The security code also show up in my MAIL inbox. All my mail goes to both. But, when I am on a financial website, and I need to check my yahoo mail inbox for the sent code, the only way I know how to do that is to leave the website, thereby defeating the purpose of logging in. In that case I would go to MAIL, to read the code, then switch back to my financial site, enter the code and then I'm in. That worked great until the following occurred: The problem I WAS having is that on some websites, when I hit to send an email to that company, it would default to (I guess) MAIL. I was not able to send those emails out. Not a major problem but what was happening was those drafts could not be deleted. Subsequently every time I went to mail, to read the code, those draft messages would flash on screen. On then off, on then off, until I quit MAIL.
After your answer, I checked my default mail program. It is MAIL.app. The only other option is MicrosoftOutlook.app or SELECTED, which takes me to my Application folder which I don't know what to do there.. Anyway all this was happening while my screen changed between the draft and the Mail program. I found the folder that had the flashing draft (3 actually) and deleted each one. Flashing stopped and everything is good now.

OK, that sounds normal. You probably don't want to use Outlook unless you really need its PIM (Personal Information Manager) ability. Anyway, glad you got it sorted.

If you want to check your mail at Yahoo without leaving a site you're logged into, just open a new tab.
Login to Yahoo, check mail, log out, close tab. Using your browser at Yahoo isn't considered a mail client. So Mail will still be your default mail client whenever you click on a mailto link.