Thanks, guys, for your quick replies. I did that, but I get the exact same mailer error in the system log.
It now also logs another error:
Unable to email via SMTP:smtp.office365.com:587 [e-mail address of the account] Failed to set sender: e-mail address of the account [SMTP: Invalid response code received from server (code: 530, response: 5.7.57 SMTP; Client was not authenticated to send anonymous mail during MAIL FROM [MWHPR19CA0079.namprd19.prod.outlook.com])]
Looking more closely through the logs, it looks like it always logged this message also. I just didn't notice it until now.