Why my NAS can't accept GMAIL as email notification account?

https://support.google.com/accounts/answer/6010255

Google may block sign in attempts from some apps or devices that do not use modern security standards. Since these apps and devices are easier to break into, blocking them helps keep your account safer.

Some examples of apps that do not support the latest security standards include:

• The Mail app on your iPhone or iPad with iOS 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

Access your account

To help keep your account secure, we may block these less secure apps from accessing your account, and you’ll see a “Password incorrect” error when trying to sign in. If this is the case, you have two options:

• Upgrade to a more secure app that uses the most up to date security measures. All Google products, like Gmail, use the latest security measures.
• Go to Allow less secure apps and choose “Allow” to let less secure apps access your Google account. We don't recommend this option because it may make it easier for someone to gain access to your account.

If you’re still having issues accessing your account after trying the options above, see other reasons you might receive the "password incorrect" error.

Notification via Gmail on OS5 & OS7

There are some necessary settings on your gmail account or the test mail will fail:
1. If using 2-step verification (enter verification code beside password at login): enable "app password" [ https://support.google.com/mail/answer/185833?hl=en ]
2. Else: enable "less secure apps" [ https://support.google.com/accounts/answer/6010255?hl=en ]