On May 30th, 2022, Google has announced that they are dropping support for "Less Secure Apps".

https://support.google.com/accounts/answer/6010255?hl=en

Basically what this means is that if you are using an application or program that sends email from  a GMail account (ie, BobSmith@gmail.com) and that application uses the account's user ID and password for authentication, well, it will cease working on May 30th, 2022.

From the announcement it doesn't apply to Google Workspace accounts (where you have your email sending from your domain but still using the Google Mail servers).  But I am sure that will come in the future.

So how does this apply to sending email from the IBM i?

Simple... if you are sending email using a basic GMail account (ending in gmail.com) you will need to find another way to send emails that uses a process such as OAuth 2.0.  I haven't seen any offerings in the IBM SMTP server that will allow this, but luckily for MAILTOOL users (and users of other 3rd party email apps that create a MIME file) they can use the GreenTools for Google Apps (G4G) Send Email Addon (G4GSMAIL).

This will require the licensing of the base G4G product, GETURI (if you don't already have a license) as well as the G4GSMAIL addon for sending emails.

In order to use the G4GSMAIL Addon to send emails with MAILTOOL, you would follow these steps:

1. Download and install GreenTools for Google Apps (G4G).
2. Apply license keys (temporary for testing, or normal keys if you've already purchased them) for G4G, G4GSMAIL and GETURI.
3. Register the account(s) that will be used to send emails from your IBM i using the G4GREGSVC (Register Service) command to register the *G4GSMAIL service. 
4. Make sure that MAILTOOL and G4G libraries are in your library list.
5. Once that is complete, all you need to do is specify the following parameter on the MAILTOOL command:
   
   SENDWITH(*G4GSMAIL)
    
6. This will do all the heavy lifting in the background for the required OAuth 2.0 functionality, including renewing any authentication tokens.

Here's a video showing that process of registering and then using MAILTOOL to send an email using G4GSMAIL.  Now, this example does use my bvstools.com account to send (which uses GMail) but this will, for now, mainly apply to basic GMail accounts, or those ending in gmail.com.  The process will still be the same.

If you wish to use 3rd party email applications or your own email software and still take advantage of the OAuth 2.0 functionality of G4GSMAIL, there are standalone ILE functions available in the G4G package.  You will simply need to have your email software pass the MIME file it creates to the function to send the email.  See this article for more information.

Are There Any Other Options?

Yes.  It appears Google also allows you to set up Application Passwords, which also requires turning on 2 Factor Authentication (2FA) but is not recommended unless necessary.  See this article for more information.  

Also see this article: How Do I Switch From MAILTOOL Plus to GreenTools for Google or Microsoft Office 365?