Thank you, @VT-Freddy! We were experiencing the same issue. After adjusting our mail filter settings, it persisted. Continued on this path and found the issue was being caused by Microsoft security settings. Tested the Meraki settings and isolated the problem to our internal mail by creating a copy of the SSID with an external email as the authorizing domain. For others finding this in a search, take a look at all of the filter/security/inspection systems that are touching your mail. Additionally, there is a Captive portal strength setting under Wireless > Access control > Splash page > Advanced splash settings to select a radio button to either Block all access until sign-on is complete or Allow non-HTTP traffic prior to sign-on.
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