Starting on January 31st, 2021 Robin will be removing support for TLS 1.1 connections to its servers for a more secure form of communication.

Who’s affected: Exchange customers that do not have their servers configured for TLS 1.2.

What’s affected: Realtime calendar syncing is not possible without TLS 1.2 communication to our servers.

Instructions: Follow the two-part guideline from Microsoft to ensure your servers are up to date and configured to use TLS 1.2:

• https://techcommunity.microsoft.com/t5/exchange-team-blog/exchange-server-tls-guidance-part-1-getting-ready-for-tls-1-2/ba-p/607649
• https://techcommunity.microsoft.com/t5/exchange-team-blog/exchange-server-tls-guidance-part-2-enabling-tls-1-2-and/ba-p/607761

After you've completed the above instructions and have verified TLS 1.2 outbound connections, calendars should sync as normal.