SUMMARY For Office 365 you can access shared mailboxes using this pattern as user name for authentication: user@company.tdl\shared-mailbox@company.tdl. With the Backslash authentication doesn't work. It works in Thunderbird. STEPS TO REPRODUCE 1. Create a new IMAP account 2. IMAP Server: outlook.office365.com, STARTTLS, PLAIN 3. Username: user@company.tdl\shared-mailbox@company.tdl OBSERVED RESULT * Authentication failes * At some point in the logs I've seen that some "SASL" line wrote "user@company.tdl\\shared-mailbox@company.tdl" as username. It seams that the single backslash gets a double backslash. I guess that's the root cause. EXPECTED RESULT * Login works SOFTWARE/OS VERSIONS Linux/KDE Plasma: Kubuntu 20.04 KDE Plasma Version: 5.18.5 KDE Frameworks Version: 5.68.0 Qt Version: 5.12.8