you can comment out auth_username_format="%Ld_%Ln" as well whole socket listen section in dovecot.conf + comment out...
Rockdrala, in my opinion you can any time revert to old setup by restoring original postfix main.cf and dovecot.conf and have everything working...
Many thanks to Torfinn Nome for this nice and much needed hack. I have only recently started using ISPConfig and just have few doubts before...
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