The UNIX program "mail", not the mail client one would use by choice under most circumstances. However today I have the pleasure of using a Solaris box which seems to be filled only with legacy tools like original "vi", a "tail" where if you do "tail -f *" it only tails the first matching file and not all files.. but I digress.
$ mail mail: Too many letters, overflowing letters concatenated
I mean ignoring the error itself and the disrespect which it forces me to show this program, look at the wording.
The error involves the word "letters", i.e. the program was written before the word "mail" was the standard word for internet mail.