Mailing lists are a natural technology for supporting messaging in multi-party, cross-domain collaborative tasks. However, whenever sensitive information is exchanged on such lists, security becomes crucial. We have earlier developed a prototype secure mailing list solution called SELS (Secure Email List Services) based on proxy encryption techniques , which enables the transformation of cipher-Text from one key to another without revealing the plain-Text. Emails exchanged using SELS are ensured confidentiality, integrity, and authentication. This includes ensuring their confidentiality while in transit at the list server; a functionality that is uniquely supported by SELS through proxy re-encryption. In this work we describe our efforts in studying and enhancing the usability of the software system and our experiences in supporting a production environment that currently is used by more than 50 users in 11 organizations. As evidence of its deployability, SELS is compatible with common email clients including Outlook, Thunderbird, Mac Mail, Emacs, and Mutt. As evidence of its usability, the software is being used by several national and international incident response teams.