When you moved to Dynamics GP 18.4 and O365, did you also implement the new Azure MFA functionality mentioned in this blog?
docs.microsoft.com/.../multi-factor-authentication
community.dynamics.com/.../microsoft-dynamics-gp-fall-2020---multi-factor-authentication
If not, as Beat mentioned, your O365 users will not be able to have MFA (multi-factor authentication) enabled on them, otherwise their credentials will not be accepted.
If you do not have this new Azure MFA implemented, when users attempt to email, they'll be prompted with the same Exchange Log On window as they've seen before when emailing in GP, where they enter their Exchange credentials. That troubleshooting is mentioned in the email troubleshooting guide you referenced.
If using the new Azure MFA, users will instead see the same type of Windows prompts for user id, password and then MFA code that they would see outside of Dynamics GP if using MFA with other applications such as Outlook, Teams, etc.
I would also try emailing using the Email button from the GP windows themselves, if you're not already doing so, as the 'Send To' > 'Mail Recipient (Text)' on reports printed to screen isn't the best test for emailing from Dynamics GP.
The first thing we'd want to verify is whether or not Azure MFA is implemented in Dynamics GP or not and then if not, whether your O365 users have MFA enabled on them.
The second thing, would be what are you actually seeing when users attempt to email in GP? Do they get prompted for their user id and password credentials, or no? If so, are they accepted or not?
Thanks
