Here's the bounce message from Microsoft, with email addresses and signatures removed:
Delivery has failed to these recipients or groups:
abcdefg@abcdefg.plus.com (abcdefg@abcdefg.plus.com)
Your message couldn't be delivered because the recipient's email server (outside Office 365) suspected that your message was spam. To fix this, try to modify your message, or change how you're sending the message, using the guidance in this article: E-mailing Best Practices for Senders. Then resend your message. If you continue to experience the problem, contact the recipient by some other means (by phone, for example) and ask them to ask their email admin to add your email address or your domain name (the text after the "@" symbol in your email address) to their allowed senders list.
For Email Admins
When Office 365 tried to send the message to the next email server outside of Office 365 the external email server returned the error below. This indicates the recipient's email server (or their email filtering service) suspected the sender's message is spam. If the sender isn't able to fix the problem by modifying their message then contact the recipient's email admin and ask them to add your domain name, or the sender's email address, to their list of allowed senders.
While the sender may be able to alter the message contents to fix this, it's likely that only the recipient's email admin can fix this problem. Unfortunately, Office 365 support is unlikely to be able to help fix these kinds of externally reported errors.
Diagnostic information for administrators:
Generating server: DB7PR06MB4268.eurprd06.prod.outlook.com
abcdefg@abcdefg.plus.com
Remote Server returned '550 5.7.350 Remote server returned message detected as spam -> 552 Spam Message Rejected '
Original message headers:
ARC-Seal: i=1; a=rsa-sha256; s=arcselector9901; d=microsoft.com; cv=none; [snipped]
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=microsoft.com;
s=arcselector9901; [snipped]
ARC-Authentication-Results: i=1; mx.microsoft.com 1; spf=none; dmarc=none;
dkim=none; arc=none
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=hotmail.com;
s=selector1; [snipped]
Received: from DB6PR06MB3928.eurprd06.prod.outlook.com (2603:10a6:6:54::10) by
DB7PR06MB4268.eurprd06.prod.outlook.com (2603:10a6:5:32::19) with Microsoft
SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id
15.20.4264.23; Wed, 30 Jun 2021 08:08:20 +0000
Received: from DB6PR06MB3928.eurprd06.prod.outlook.com
([fe80::f0eb:b751:4700:d5f8]) by DB6PR06MB3928.eurprd06.prod.outlook.com
([fe80::f0eb:b751:4700:d5f8%3]) with mapi id 15.20.4264.026; Wed, 30 Jun 2021
08:08:20 +0000
Moderator's note by Mike (Mav): Post released from Spam Filter.
