550 5.7.515 Access denied

The rejection is caused by sender-domain authentication, not the Outlook port or password. Microsoft’s 5.7.515 response means the domain in the visible From address did not meet Outlook.com’s required authentication level. Your result already shows the key clue: DKIM fails. Webmail or Gmail can work because they may use a different outbound server, DKIM signer, return path, or From domain. Send one test from Outlook and one from webmail, then compare their Authentication-Results, From, Return-Path, and DKIM d= values. Ask the ISP or domain administrator to enable valid DKIM signing on the SMTP service used by Outlook and verify that its domain aligns with the From address. Also confirm SPF includes that outbound service and that a valid DMARC record exists. Port 465 versus 587 will not repair DKIM. If you do not control the domain’s DNS or mail server, only the provider can correct this.