When trying to send an e-mail from Roundcube Webmail in Hestiacp, the following error appears. SMTP Error (): Connection to server failed

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If possible, it is recommended that you reinstall Hestia and related mailbox components! Hestia doesn’t work with Postfix

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Please wipe the server and start from zero. As @sahsanu already wrote, you had some Frankenstein there and it’s impossible to support.