This article will help you resolve the "Working Offline," "SSL/TLS," or "Cannot connect to remote server" errors. These errors are usually caused by a proxy or firewall blocking the URLs that the Microsoft Outlook client needs to connect to to verify our SSL certificates' validity.
You may also need to allow these addresses through any firewalls or proxies where packet inspecting occurs. Packet inspection can impact the connectivity and performance of the Microsoft Outlook client service.
The issue can be resolved by whitelisting the following URLs:
- http://crl3.digicert.com/DigiCertHighAssuranceEVRootCA.crl
- http://cdp.geotrust.com/GeoTrustEVRSACA2018.crl
- crl.usertrust.com
- ocsp.usertrust.com
- ocsp.sectigo.com
- switch.egress.com
- http://gc.egress.com
- lencr.org
Depending on your Protect location, you will also need to allowlist your region:
- Us1.esi.egress.com
- Au.esi.egress.com
- Eu1.esi.egress.com
Ports 80 and 443 should also be open to allow access to switch.egress.com.
Prevent
If you are a Prevent customer, add the following exception:
- *.intelligence.egress.com
Large File Transfer
Large file transfers are uploaded to Amazon S3 (UK) by default. For the right URLs to allowlist according to the location of your Amazon instance, read Amazon's Service Endpoints Support page.