poll mail.domain.com
    protocol imap
    username "email@domain.com"
    password "password"
    options ssl

department1@domain.com -> email@domain.com
department2@domain.com -> email@domain.com
department3@domain.com -> email@domain.com

fetchmail gets email@domain.com

email@domain.com "To: department1@domain.com" -> postfix alias 1
email@domain.com "To: department2@domain.com" -> postfix alias 2
email@domain.com "To: department3@domain.com" -> postfix alias 3