Safe Links is a feature in Office 365 that provides URL scanning and rewriting of inbound email messages in mail flow, and time-of-click verification of URLs and links in email messages, Teams messages, and other locations. 

The Firstup Engineering team has found that enabling Safe Links in Office 365 will impact the following functionality:

• Authentication tokens - Link tokens are already expired upon email message delivery.
• Metrics - Open/click metrics are skewed in Insights.
• Traffic - Large spikes of traffic are seen on the Firstup system.

While Firstup can't prevent these impacts, we always recommend allowing Firstup domains and IPs to support successful email delivery.