I personally use a 64 bit kernel and 32 bits userland on my servers. I don't /need/ 64 bits for most things, and the 64 bits kernel gives me quite a bit more addressing space if I need it.
However 64 bit userland is about 15% faster on the same hardware. It's also 15% bigger in general.
As for email, many many years ago I decided to become free of the email "client of the day/year/decade" and have to "convert" etc.
What I now run is a small "dovecot" linux IMAP server that receives all my mail (via procmail) and I use the "client of the day" to connect to it. This allows me to use as many "clients" as I like (iPhone, iPad, webmail etc) and the mail storage is stored in simple "mbox" file that are easy to index, backup, and are in a file format that has been universally used for 25 years.