In Version 8.5.2 or above there is a way to set a default font for new mails by using the menu "Text - Set current Font as mail default" as described here.
Unfortunately this feature is not available in 8.5.1 so that setting the default font there would need designer access to the template / mailfile. Using Domino Designer you could set the desired font and size for the Body field in the three mail- forms Memo, Reply and Reply with History.... But usually a normal user does not have Designer installed and does not have the necessary access to his mailfile.
Setting the default fonts as suggested in the other answer most probably will not really help, as it is just used for displaying, the font "properties" for your sent mail will still be "Default Sans", just the way it is displayed within your client will be different...