No. SMTP can be setup to use ports other than 25, although that is the standard port used.
SMTP is a protocol – it specifies HOW information is transmitted. The SMTP Server, on the other hand, is software (separate from the protocol itself) that does the transmission. The SMTP port is actually a listening port – clients connect to this in order to use the server – but it's separate from the protocol itself. Many mail servers are setup to use other ports. For example, Dreamhost sets up its mail servers to use both port 25 and 465.
