Mailwatch

The Mailwatch plugin, for Xubuntu, allows checking the email box without opening it manually.

Install it with:

sudo apt install xfce4-mailwatch-plugin

Add Gmail account

Important: The Gmail account should be set to allow the access to less secure apps (as described in Gmail requires OAuth 2.0 and in Allow less secure apps to access Gmail account).

Add the Gmail account filling the form as follows:

Mailbox Name	name of recognition
Username	Gmail user (without `@gmail.com`)
Password	Gmail account's password
Check for new messages every (minutes)	`1`

Fill the form as follows:

Run on click	`firefox https://gmail.com`
Run on new messages	`notify-send -i xfce-newmail "Gmail" "you've got mail"`