Port 57 vs 587

At my office:

OK (out-going mail): telnet servername 25
OK: telnet servername 110

At my client's office:

No: telnet servername 25
OK: telnet servername 110

On advise of the web host, at client office change port 25 to 587:

OK: telnet servername 587


Question: Why is this so ?

Thanks.
.