The following guidance is provided as-is and without any warranty to resolve unexpected mail rejections from recipient environments that use the UCEPROTECT service as a block list.
To resolve this issue, try the following. If the recipient messaging system uses the UCEPROTECT block list, contact the recipient mail administrator to have the following Exchange Online Protection outgoing edge server IP addresses added to an allow list to bypass the UCEPROTECT checks:
Microsoft is committed to enabling customers to have a secure email environment that is both spam-free and virus-free. As part of that commitment, Exchange Online Protection takes many steps to make sure that mail that is filtered through our network doesn't contain unsolicited commercial messages.
Article ID: 2926328 - Last Review: 28 Dec 2016 - Revision: 1