All email scripting variables are String objects which can manipulated by methods here - http://docs.oracle.com/javase/6/docs/api/java/lang/String.html
Below is the modified code that will work:
#set ( $code = ${lead.PostalCode} ) ##set the variable code to equal to the lead's postal code. This is not necessary. Just a personal preference to shorten the name
#if ( $code.startsWith("1") )
salesrep1@company.com
#elseif ( $code.startsWith("2") )
salesrep2@company.com
#else ##any other leads that don't match
callmemaybe@company.com
#end