Mail address sieve and Exchange

During a discussion about fighting spam, a coworker mentioned an interesting Gmail feature. As that service doesn’t provide aliasses, you can’t have a specific mail address alias for your internet services. Some use a different email address for each service they sign up for, a practice which is understandable with some services that (sometimes illegally) opt-in bulk mailings or more nefarious mailings. Whenever a specific alias receives unwanted mails, you can see where it came from and take direct action like complain, create a rule with a fitting action or just disable the alias.

Gmail offers another solution, namely mail addresses with sieve. Instead of having an alias, you can add an extra text portion to the localname part of the SMTP address, that is the part before the @. The delimiter is the + symbol. So, if your address is, an sieve address would be Despite the +sieve, the mail will still be received by Benefit is that the user doesn’t have to prepare anything, such as with aliasses, resulting in an almost unlimited amount of mail address variants.

Although Gmail doesn’t mention this explicitly, this feature is described in RFC 5230, with some additional information in RFC 5228. And how about Exchange? Nowhere is there explicit support stated for this function and testing with Exchange 2013 on-premises and Online with Exchange Online Protection (EOP) resulted in NDRs, with the report that the recipient isn’t known. Is that bad? I never have had the request for this feature before, I even didn’t know that it existed.

Personally I don’t think enterprise organizations would like the idea that users could use an unlimited variant of their mail address without admin control. Especially for signup on services that probably don’t benefit the goals of that organization. I can imagine that fighting spam/malware would be more of a challenge with sieve addresses, as for instance recipient filtering would have to take this feature into account.

If you don’t want to use Gmail (for whatever reason), it seems as if the non-enterprise service (previously Hotmail, Live, MSN whatever), does support sieve addresses although I can’t find a source confirming this (other than myself testing it). I like the general idea behind sieve addresses, but in my opinion I find this more fitting with consumer grade mail services. It seems as if Microsoft shares that opinion.

Update: If you really require this functionality, Exclaimer does provide this with their tool Address Tagging for Exchange. Thanks to fellow Exchange MVP Michel de Rooij for this tip!

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