When sending mail as an alias from eM Client using EWS, the Return-Path header is being set to the login email address as opposed to the alias when the email is sent outside of SmarterMail. When sent to another domain on the same SmarterMail server, the X-OriginalSender is set to the login email address. When sent to the same domain on the same SmarterMail server, the login email address is not displayed in the header.

On the contrary, when sending mail as an alias from both Webmail and BlueMail via EWS, the received email correctly shows the Return-Path or the X-OriginalSender as the alias.

In the user's sent folder on the SmarterMail server, the original sent email does not show the login email address anywhere in the header, so it seems that SmarterMail (as opposed to eM Client) is adding this as the email is being sent.

I'm currently on Build 9294.