mail vs. mailx: Berkeley mail users may note that when they type "mail" on a Solaris system, the Subject: prompt does not show up. On all Solaris systems, you should not use "mail", you should use "mailx". The vendor did it this way to be compatible with other System V-based UNIX systems. On System V, "mail" has a certain set of parameters and a certain operating mode that is not compatible with the Berkeley (SunOS) "mail". Sun has compromised and has "mail" compatible with System V, and gives us "mailx" which is compatible with the old SunOS Berkeley "mail". People who used to use "mail" as a mail reader probably also want to use "mailx".
The "elm" command should operate the same on both systems since it is third party software.