Update Gmail App Server Settings

Open the Gmail app on your device. 

Click the menu icon to the left of the search field.

Scroll down to the bottom of the menu and click Settings.

Click on the account you wish to alter

Scroll down to the bottom of the settings and select Incoming settings.

Ensure the following settings are used:-
Username: *Your full email address*
Password: *Your email account password*
Server: imap.stackmail.com
Port: 993
Security type: SSL/TLS
Then click Done.

Then click on Outgoing settings.

Ensure the following settings are used:-
Require sign-in: On
Username: *Your full email address*
Password: *Your email account password*
Server: smtp.stackmail.com
Port: 465
Security type: SSL/TLS
Then click Done.