Hi my name is Jason Meers. I am Head of IT for one of the largest privately owned companies in the UK. My users are all Windows/Office/Exchange users however 80% of my network runs on Linux. Approx 95% of our workstations run the "ThinStation" thin client operating system (which I help develop; mostly fixes to other peoples code) to connect to Citrix or Win2003 Servers. All of the "glue" in my network runs on CentOS on mini-itx hardware or disk-on-chip. Our exchange servers are actually Scalix running on Centos (but you would never know) and all of the e-mail routing for the UK is done with Exim on CentOS (I run a website for Exim users, write tutorials and have written a book about opensource e-mail which is being published this year). The NCC (National Computer Centre in the UK) used my network as a basis for some of their papers on Linux which led to successful implementations of open source in UK local government.
I will be doing a lot of work with SMTP/IMAP/POP3 over the next few months and will be producing a CentOS based minimal-mail-server-cd with Karanbir later this year. I also have hardware that needs better QA this time round (It was impossible to boot/install some versions of 4.x on VIA EPIA mini-itx hardware, and 3-Ware escalade RAID controller support stopped working in some versions of the 2.6 kernel last year). I have also just become an author for a UK based Linux magazine.
Thanks Jason_Meers -- http://www.exim-new-users.co.uk