This is an old thread, but I've recently had problems sending mail while setting up a couple of new Macs using Snow Leopard.
Typically, after using the Mail setup assistant, one can receive emails but cannot SEND them out. Quite often, an alert will pop up saying that the password entered was incorrect -- even though you KNOW it's the right password.
I believe the error may not be localized to Mail.app itself, but may actually have something to do with the way the Mail setup assistant works, or also how the password "key" is created in the user's keychain.
I'm sensing that when the Setup Assistant creates the new mail account, and creates the password key, that _something_ is being "created amiss" (that is to say, "not quite right"). Thus, when Mail.app invokes that key in an effort to send mail, there is a conflict with the server/send process and an error is thrown up.
This may explain why "turning authentication off" seems to correct the problem -- I'll guess that with authentication off, Mail.app is no longer using the password key stored in the keychain and instead waiting for manual input from the user.
Here's what to do to fix the problem:
- Open the Keychain Access application in the Utilties folder.
- Unlock your keychain by clicking on the lock icon in the lower left
- Locate the "passwords" that are stored in the keychain, and DELETE ALL that are related to the Mail.app (should be indicated by "POP" and "SMTP").
- Quit Keychain Access
- Now, re-launch Mail
- Create a new "test email" to yourself
- When you click send, the SMTP server will request a password, which will invoke the dialog asking for the password and if you wish for it to be stored in your keychain. Enter the password and CHECK the box to store the password.
- Does it send succesfully, without an error?
- If so, now choose the "get mail" option to receive the email to yourself.
- Again, since there is no stored password, you will again be prompted to enter a password for the "POP" server. Enter it and again check to store it in your keychain.
This got things working properly for me again.
NOTE: It may be necessary to go into the accounts pref pane in Mail.app, and adjust your server ports, as well.