Lotus sells an SMTP gateway, but the latest 1.1 version should be used (1.0 had a lot of quirks). The 1.1 version still has numerous bugs. More is known about the Lotus SMTP gateway than the other products below.
TenFour Systems from Sweden sells the TFS Gateway.
Information Electronics has a product called PostalUnion which has a centralized post office that can be interfaced to various mail systems. They have Notes and SMTP modules.