Fix re-test in secure Icedove autoconfig wizard

Originally created by Tails on #6157 (Redmine)

research how to fix the "Re-test fails in the account creation wizard manual configuration window": Icedove ignores proxy settings, tries to connect to the Internet directly, and is forbidden to.

Parent Task: #6154 (closed)

Related issues

Related to #6369 (closed)

Edited May 15, 2020 by import-from-Redmine

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