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 safe.
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
Step-by-step guide if you want to allow less secure apps:
Login to your MountaineerApps email account.
Click on your photo (if you have not set a photo you will see a blue person)
Select "My Account"
On the left hand column select “Connected apps & sites”
At the bottom of the window set the following: Allow less secure apps: ON
Authentication in Outlook, Thunderbird and other apps will now work.