As of Oct 1, 2022, Microsoft rolled out new plans for addressing security issues related to email and browsing. There was a release of TLS1.2 which deprecates previous browsing through SSL that was 20 years old.
Additionally, for email, multifactor authentication has essentially been deprecated for what is now referred to as Modern Authentication.
Starting in GP18.3.1173 it is required to do the following steps in order to begin successfully emailing through GP
Here is the overview:
1) In Office 365 as a global administrator, you will need to 'All Admin Centers' - Azure Active Directory Admin Center. Once there, click on 'Properties'. On the bottom of the window you will see 'Manage Security Defaults'. This will need to be enabled.
2) Under App Registration still under the Azure Active Directory Admin Center, you will see your Application Client ID. This ID needs to be copied into GP under Tools-Setup-Company-email settings. The box will need to be populated for each company using email functionality.
Here are some guides provided by Terry Heley in case there are errors or problems
Link to configure Multi Factor Authentication
Other links