Security Defaults blocks legacy authentication
Microsoft documents that Security Defaults blocks legacy authentication protocols. A workaround that depends on reusable Microsoft account credentials conflicts with that security direction.
Do not disable broad protections for one device
First establish whether the client supports OAuth or Graph and whether another Microsoft sending method fits. Avoid making a tenant-wide policy exception before documenting the business requirement and safer alternatives.
A bridge changes where modern authorization occurs
With smtpify365, the user authorizes delegated Graph access at Microsoft. The device authenticates only to the bridge over STARTTLS with separate generated credentials; it does not authenticate to Microsoft with the mailbox password.