For me nothing has changed after the update regarding SMTP over port 465, but I don’t use MFA for my email accounts.
Did you set up MFA for these accounts before or after you updated to 2025-03? Or let me ask a different question, did sending email ever work after you set up MFA?
If yes, then I’m not sure what caused this.
If no, then you need to set up app passwords for these accounts. This can be done by logging in to the accounts through the user login page (mail.yourdomain.tld without /admin), then go to the tab “App Passwords” and click on “Create App Password”. Alternatively, for Apple devices, you can download a profile file from the “Mailbox” tab.