Under 10.10.2, Mail is currently version 8.2. My Mail iCloud account is setup as IMAP; and the SMTP settings in my iCloud account's SMTP set-up are as shown at this Apple Support link iCloud's Mail Settings .

While I don't have any problems sending & receiving email out of my iCloud account, Mail's Connection Doctor reports that while my iCloud IMAP connection and login succeeded, it reports iCloud SMTP "Trying to log in to this SMTP account failed. Verify the username and password are correct."

I've tested sending emails from my iCloud account to a Yahoo Mail account, accessed from the web (the Yahoo account is not set-up in Mail), and the test email is received without problem at my Yahoo Mail. ...This would seemingly indicate the SMTP settings in my iCloud setup are working.

I've re-verified the iCloud account SMTP settings (SMTP server address, login name - full email address, password), but Connection Doctor continues reporting the same SMTP login error.

Any thoughts as to why Connection Doctor reports the error regarding iCloud SMTP?

Have you looked at the Console log files? Yu might find more information about what is going on there.

Looked around within all the various, Console logs, and I didn't see anything seeming related to Mail.

But to be candid, and as there's so much technical detail in the logs, I'm not sure if I'm overlooking something.

There is a search function in Console that might help. However a few thoughts…

1. Normally the iCloud account settings in Mail are generated automatically when you specify your Apple ID and password during this initial Yosemite setup or later in System Preferences > iCloud or in Mail > Preferences > Accounts. Mail knows the correct settings for iCloud and you should not have to make any manual adjustments or tweaks for that to be correct.
2. Since you are able to send and receive messages using the iCloud account it would appear that your current settings are correct.
3. Sometimes any email account, even iCloud, can be slow to respond when queried by an email client which can result in an individual query timing out and/or throwing and error message. Previous versions of Mail has often been criticized for being overly sensitive about this. Frankly I don't know about the current version of Mail as I switched to Airmail about the same time I installed the Yosemite beta.

With nothing else to go on, I would speculate number 3 is the source of your problem. If that is the case there is nothing you can do except grin and bear it.

Based upon Mail being able to send out of my iCloud account, I think your #3 is close to the mark.

It just seems strange that Connection Doctor can't correctly report on the iCloud SMTP query, if it's timing out, when all the other queries quickly complete and verify satisfactorily.

So, I'll just go with the flow...

The error condition that is reported back from a remote server or the client can derive from a specific situation often does not contain enough information to determine one error condition from another which results in misleading or uninformative error messages. The situation described in #3 has been around in Mail since the OS X 10.0 and before. In fact it is not even truly an error rather it is a misreporting of a not uncommon occurrence that in some cases could be an error. Adding Connection Doctor on top of that simply creates the opportunity for a different erroneous error message. It probably occurs in other email clients, but rather than report it, they may either ignore it or simply create a log entry in case it really is a problem.

Mail 9.0 (in El Capitan) has fixed this issue. ...Connection Doctor now reports iCloud SMTP connection was successful!