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The procedure for importing the HTTPS proxy certificate into a mobile device is described in this Knowledge Base article as of step 3.4.
Manual download of the Mail Proxy CA (Optional):
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Open the the web interface of the Unified Firewall and go to the menu Certificate Management → Certificates, klick on the Export button at Mail Proxy CA and export the certificate in PEM format. The procedure for the certificate import into an end device is described in this Knowledge Base document (as of step 3.4).
