Multi Factor Authentication for Emails

Infusion does not support Multi Factor Authentication (or two factor authentication) for Emails (MFA) or 2FA

The following is from the Chilkat website (the DLL we use)

smtp.office365.com: If SMTP authentication fails for your smtp.office365.com account, it may be that your account is configured to require MFA (multi-factor authentication). You may need to change settings to allow for legacy authentication (single-factor auth) See Here

Also, an app password may be required.  see here

The Chilkat socket component/library provides a high-level, easy-to-use API for TCP/IP socket programming. It supports creating secure channels w/ TLS 1.2, 1.1, 1.0, and SSL 3.0. Both client-side and server-side SSL/TLS is supported, 

Lastly ensure that the Use TLS tick box is selected in Infusion Email Defaults.