Roundcube, Microsoft Exchange Server, Zimbra Collaboration Suite, WikiSuite, Greg, wtplan, OBM, CalenGoo, Nextcloud Hub, Kopano, Calcurse, IceWarp, CrossBox, Attendexer, Contactoffice, GMX Calendar, Expanding Communication, SMBMail, Sanwhole Exchange, KeepTime, EGroupware, Calendarific, Postaci, MailShogun are some alternatives to Kolab Community.