Emails from domains that are not on the Outlook Safe Senders list may have a warning displayed on them, as well as some of the email content including images blocked.
To prevent the 'Some content in this message has been blocked because the sender isn't in your Safe senders list' message from showing up, you will need to add usecure domains to the Outlook Safe Sender lists of each of your end users.
How to add usecure domains to your Outlook Safe Sender lists
You will need to run a PowerShell script to add usecure domains to the Outlook Safe Sender lists of your end users.
Important note: You will need to run this script every time you add new users - or each time before you run a simulated phishing campaign - to ensure all users have usecure domains added to their Safe Senders list.
Here's what the script looks like. You will need to do this for each usecure domain.
$All = Get-Mailbox -RecipientTypeDetails UserMailbox -ResultSize Unlimited; $All | foreach {Set-MailboxJunkEmailConfiguration $_.Name -TrustedSendersAndDomains @{Add="usecure.io"}}
Download the PowerShell scripts for each domain in this text file
Run each script in PowerShell
Check that the usecure domains have been added to users' mailboxes
Credit to Will Silva at SPC IT Limited for finding this solution.