You are running Data Loss Prevention (DLP) Endpoint Prevent with the Outlook plug-in enabled. Users are reporting messages with a large number of recipients are taking longer than usual to send.

The delay occurs due to the need for DLP to enumerate each recipient against the DLP policy set. 

This is expected behavior.

It is recommended to create a distribution list instead to work around this issue. Note: Users should be instructed to not expand the distribution list as this will cause the issue as well.