This happens most often when you change your Gmail password.
Gmail automatically deletes all your app passwords, including the one used in MCA Pilot.
So the solution is simply to create a new app password.
Go to the Manage your Google Account page > Security > 2-Step Verification page.
Scroll to the bottom and click the arrow next to "App passwords."
From there you can type any name for the password; if unsure, use "MCA Pilot" so it's identifiable in the future.
Click the
Createbutton and copy the 16-character password to clipboard (make sure not to copy an extra space at the end).
On the Settings > Senders page hover over your Sender and click the
Update Passwordbutton.Paste in the app password and click Save.
Hover over the Sender again, click the
Test SMTPbutton, and enter any email address to send the test to.If it says "Verified", you're good to go!