Catalyzes the methylation of the free carboxyl end of the plant hormone salicylic acid (SA). Converts SA to SA methyl ester (MSA). The volatile compound MSA is hypothesized to act as an airborne signal that triggers defense responses in uninfected plants. MSA is an important chemoattractant for moth pollinated flowering plants. S-adenosyl-L-methionine + salicylate = methyl salicylate + S-adenosyl-L-homocysteine Belongs to the methyltransferase superfamily. SABATH family.