Why is my Gmail SMTP settings not working?
What are the Gmail SMTP settings? That is an easy question.

Gmail is a great webmail app, but some people find it more convenient to access their own inbox from an email client like Thunderbird or Outlook - this requires setting up your outgoing server by entering the correct settings.

First, open the configuration panel of the mail software:
and fill in the fields with the Gmail SMTP server settings, which are:

Gmail SMTP server name: smtp.gmail.com
Gmail SMTP username: your Gmail address
Gmail SMTP password: your password
Gmail SMTP port: 465
You can then choose your security and authentication options - for more information, see our article on SMTP settings. (And of course you can also use Gmail's POP details to receive emails.)

However, remember that Gmail's SMTP comes with severe sending limits to prevent spammers from using your outgoing server to delete junk emails. The limit is 100 recipients at a time and 500 messages per day: if you cross this restriction, your account will be blocked by Google.

In addition, your SMTP server is based on shared IPs that cannot guarantee you full deliverability, as they can also be used by spammers who seriously damage your reputation, thus increasing the possibility that your legitimate email will also be filtered by anti-spam systems. .

SMTP Settings - How to Customize SMTP Settings
SMTP stands for Simple Mail Transfer Protocol and is a set of commands used to authenticate and direct the transfer of email from one server to another. Your MINDBODY site is configured to send all automated emails from Business @ mindbodyonline.com by default.

However, it's easy to customize your SMTP so that your automated emails come from an email address of your choice. Not only does this mark emails with an address that your customers recognize, but the more personalized your emails are, the better they should be at getting past your customers' spam filters.

Gmail SMTP Settings
Security settings
You must change the security settings of your Gmail account from which you want to send email.

Here are the steps to change your Gmail account security settings:

Go to your Gmail account settings (gear icon) as shown in the image below and click on Other Google Account Settings.
Now click on Security in the left menu as shown in the following image:
Then click Turn on access (not recommended) to access less secure applications.
Click to activate the highlighted area in the image below:
Now you will be able to send the email using Gmail's SMTP server without any error.
It's about reducing the security settings in Gmail.
https://www.not-working.co/gmail-smtp