Due to Mail security settings, smtp testing may not be successful.
For some users, it is possible to use smtp only with low security settings.
Turning on 'less secure apps' settings as mailbox user
Go to your (Google Account).
On the left navigation panel, click
On the bottom of the page, in the Less secure app access panel, click
Turn on access.
If you don't see this setting, your administrator might have turned off less secure app account access (check the instruction above).
Turning on 'less secure apps' settings as mail domain Administrator
Open your Google Admin console (admin.google.com).
Under Less secure apps, select
Go to settings for less secure apps.
In the subwindow, select the
Enforce access to less secure apps for all users radio button.
(You can also use the
Allow users to manage their access to less secure apps, but don't forget to turn on the less secure apps option in users settings then!)