Devices/servicing polling for email in a mailbox will be affected for sure though. Some other partner I have talked with considered using a SMTP relay service for submission (e.g. Scanners - if scanners are using SMTP delivery to a mailbox (like an external sender) it will not be affected by this change, as mentioned in the blog. And also for end customers it is not mandatory. Patrtner Center Security requirements do not require AAD Security defaults to be enabled, they 'just' require MFA for every account. This leads me to the question, why do you want to enable AAD Security defaults if this causes problems for you and you had a working solution before? However, if you used app passwords before, it means you already need to have sufficient licenses to be allowed to do that, so you can - as long as Exchange Online suppo rts this scenario - still disable AAD Security defaults and use app passwords like you were used too. Azure Security Defaults is named for what it doessetting often overlooked defaults. So you can still set app passwords with AAD Security defaults on, but they will not work because of this block. Because Azure AD Security Defaults will block legacy protocols - so even when an app password was set, the security defaults will block access using protocols where app passwords can be used for.

Azure security defaults