Since legacy authentication is now disabled for Exchange Online, many clients have setup modern authentication in GP. They are finding that they can no longer use the "send to" option to email. There is a workaround, but it would require a process change for our clients that they may find time consuming. Can you update the "send to" option to work with modern authentication with Exchange Online?

https://learn.microsoft.com/en-us/dynamics-gp/installation/email-troubleshooting-guide#mfa---multi-factor-authentication-modern-authentication

When you have modern authentication enabled and you try to use the "SEND TO" option in Microsoft Dynamics GP, it will still prompt for the Exchange login. Modern authentication is not enabled off the "SEND TO" button/option. An alternative workaround to this is use the Report Option Windows as modern authentication is enabled there. For example if you are printing a Trial Balance, go to Reports | Financial Trial Balance and create a report option from this window for the report to email and modern authentication is enabled in all Report Option windows. Many customers use this for posting reports too so it will be a process change to use the Report Options window where modern authentication is now enabled VS the "SEND TO" option.