SMTP blocked when using direct send or SMTP relay

When attempting to set up direct send or an SMTP relay (i.e. sending email without using SMTP authentication), you may encounter the SMTP error message: 

550 5.7.1 Service unavailable, Client host [a.b.c.d] blocked using Spamhaus

If the IP address (or range is static), you can allow the connection by adding the IP address to the connection filter policy under the IP Allow List in Anti-Spam policies on the Microsoft Defender portal.

If the IP address doesn't show in the allow list after adding via the UI or you prefer Powershell. Connect to Exchange Online Powershell and run

Set-HostedConnectionFilterPolicy -Identity Default -IpAllowList IPAddressOrRange

This may take a few minutes to take effect.