The following companies/people offer consulting services for Dovecot:
Timo Sirainen - Primarily development of features intended for public Dovecot releases. Possibly other things.
state of mind - Fully-Featured Mailsystems and Groupwares relying on Dovecot as well as Anti-Virus and Anti-Spam-Gateways. We speak German and English.
Nci GmbH - Mailsystems on Dovecot/Postfix Antivirus and Spamgateways. Server Housing, Isp Munich, German and English.
Robert Schetterer - Mailsystems on Dovecot/Postfix Antivirus and Antispam-Gateways, Webserver Groupware etc. Linux Consulting, German and English.
iRedMail Project - Free, open source mail server solution, uses Dovecot as POP3/IMAP server. Works on Red Hat, CentOS, Debian, Ubuntu, OpenSuSE, FreeBSD.