I am trying to set up Gmail account in Kmail.
I have this message

The server for account "Google Mail (msh2nfrou)" refused the supplied username and password. Do you want to go to the settings, have another attempt at logging in, or do nothing?

Login failed, server replied: A000002 NO Invalid credentials ( Failure ) [ AUTHENTICATIONFAILED ]

everywhen I try to create.

I enable IMAP in my gmail setting and Turned Off less security app setting in Google account setting.
Please suggest what I am missing. I have been googling all day with on luck.

solved.
sorry, I thought "Turn Off less security apps" means that allow less security apps to connect to Gmail. But it is opposite.
I just neet to enable to allow less security apps in google myaccount setting.
login into gmail, click on profile logo circle at top right and go to "My Account" >> Sign-in & Security >> Connected apps & sites and scroll down.
then set "Allow less security apps: ON". done!

The answer on here is old and does not work anymore.

The new way of doing this is by creating an "App Password" using the link below.
https://support.google.com/accounts/ans ... ockCaptcha

This is used for applications which:
- Don't look secure by Googles standards.
- Google accounts with 2-Step-Verification.

Summary of instructions:
1. Use link above.
2. Click "sign-in" button on top right corner of page.
3. Sign-in.
4. Click 2nd section called "How to generate an App password".
5. Click "Visit your App passwords" link in step 1.
6. Select the following two fields from their drop-down lists 'Select app = mail' and 'Select device = Other(I put KMail)'.
7. Click "GENERATE" button.
7. Copy 'App password' in yellow and paste this where you would put your GMail password into KMail.
8. Click "Done" button on the 'App Password' page.
Done

I hope this helps as this is the first search result when searching for this KMail issue on Google.

Note: the newer version of KMail shipped as part of Applications 17.04 will not require the "app password" method anymore. It will use the authentication method which is preferred by Google (the one where you need to authorize applications).

I know this is a very old thread but I had the same problems as well. Googling for answers I found this thread.
I managed to solve it just now by accepting the 2-step verification GMail offers. Just follow what they send you and do what is asked and it will work. At least for me in KDE 5.12, Kontact 5.7.2.

[EDIT} After a day or two it was back to not being able to send mails.

As mentioned before:

This is used for applications which:
- Don't look secure by Googles standards.
- Google accounts with 2-Step-Verification.

What You have to do is:
1. Go to: https://myaccount.google.com/apppasswords
2. Chose "Other" and name it somehow e.g. "KMail"
3. Click Generate
4. Copy generated password to your KMail as user password (without spaces) and sign in

Enjoy

This should no longer be needed - KMail supports the native (OAuth) gmail authentication with both IMAP and SMTP.