Bit of a fringe case issue here...

When sending an external mail to a Mailing List that is set to Allowed Posters of "ANYONE" or "SUBSCRIBERS ONLY" when using an Incoming SmarterMail Gateway the message bounces with a NDR response of "550 No Such User Here". Internal emails that are *NOT* routed through an Incoming SmarterMail Gateway are able to be sent to the Mailing List address without error.

For example: 

exampledomain.com has MX records pointing to the Incoming SmarterMail Gateways at mx.exampledomain.com and mx2.exampledomain.com, while the primary SM installation is at mail.exampledomain.com. Both Incoming Gateways are set to SmarterMail Gateway Mode and use WebService User Verfication to https://mail.exampledomain.com. There is a Mailing List on the primary SM installation at mail.exampledomain.com called allstaff@exampledomain.com that is set to Allowed Posters: Anyone. If you send an email from someone@someotherdomain.com to the Mailing List allstaff@exampledomain.com both mx.exampledomain.com and mx2.exampledomain.com reject delivery with a "550 allstaff@exampledomain.com No Such User Here". If allstaff@exampledomain.com is setup as an Alias then the Incoming Gateways accept delivery.

Not sure when this started happening, but according to the Logs it appears that the last time the Incoming Gateways successfully delivered external emails to Mailing Lists addresses was when we were on Build 7188. So, it looks like the WebService User Verification for Incoming Gateways in SmarterMail Gateway Mode is no longer including Mailing List addresses in Build 7242.

I've tested deleting the Mailing List and re-adding it as an Alias and it works fine when sending external mail to the exact same address when routed through the Incoming SmarterMail Gateway, so this will work for the time being as a work-around solution. (Not entirely sure why some Domain Admin users setup email groups as Mailing Lists instead of Aliases to begin with, but apparently we have about a dozen domains that did.)