If you are down to 3.2 Gb, the extra amount needed is
- diskspace for storage of mail
- temporary space for package download, unpacking and installation
- temporary space for email buffering
- mail log during the day (after a log rotate, the mail.log/other log files grow with plain text); at the second next logrotote, this file is zipped.
It’s all about the use of your mail system.
What’s your purpose? If you want to install this on - for example - an 8Gb SD Card, this will probably be sufficient, but … The amount of write actions will destroy such a small card in a small period