Earlier today, I couldn't add GMail account to my Windows 10 Mail app. Nothing was wrong with my GMail password but the Mail app keeps telling me to try again. After several trials without success, I googled it and during my research, I discovered that GMail sees the Windows 10 Mail app as a less secure app.
Less secure apps or devices are blocked by Google when anyone tries to sign in from such apps or devices. Because, according to Google, these apps and devices are easier to break into, blocking them helps keep your account safe.
Some examples of apps that do not support the Google's latest security standards include:
The Mail app on your iPhone or iPad with version 6 or below
The Mail app on your Windows phone preceding the 8.1 release
Some Desktop mail clients like Microsoft Outlook and Mozilla Thunderbird.
So, What's The Way Out?
Use GMail app for your emails
Change settings of your Google account to allow less secure apps into your account.
How To Change Google Account Settings for Less Secure Apps
Google doesn't recommend this option because it can make it easier for someone to break into your account. If you want to allow access anyway, follow these steps:
Go to your Google Account. On the left navigation panel, click Security. On the bottom of the page, in the Less secure app access panel, click Turn on access.