Email configuration: outgoing email (smtp), best practices?

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  1. how about outgoing emails? now every email account has an option to define it as outgoing (smtp).
    what are the best practices for this?

  2. Is it best to have just one email account configured for outgoing email (smtp),
    or are there reasons to use more outgoing email accounts?

  3. when is it better to reply from the user’s email address, and when not?