I have a forum powered by Xenforo. We used the Enable variable bounced email address values and it worked fine in SM 15.

Sample return address would be -Return-Path: forum+8c91ab0b+websiteworld=gmail.com@fordthunderbirdforum.com

Now that we've upgraded to SM16, the forum won't sent emails out when that is checked. It tries to send the message locally and says no such user here. If we uncheck, it works fine. Really need to get the variable bounced email address values working again.

[2018.08.14] 18:51:33 [64.34.195.104][66328843] cmd: RCPT TO:<websiteworld@gmail.com>
[2018.08.14] 18:51:33 [64.34.195.104][66328843] rsp: 550 <websiteworld@gmail.com> No such user here

If an email cannot be delivered, the bounce notification will be returned to this email address. If left blank, bounced messages will be returned to the default email address. A value is required here if automated bounce handling is to be enabled. Note that this option may not work unless the return path parameter is enabled or mail is sent via SMTP using the Email Transport option.

•  Enable variable bounced email address values

  If enabled, sent emails will include the recipient email address in the bounce address field. This enables more accurate and more secure automated bounce email handling. If using automated bounce processing, enabling this option is strongly recommended.
  
  This option requires that the specified bounce account is a catch-all account or supports a "+" as a wildcard separator (such as in Gmail). For example, if this option is enabled with a bounce address of bounce@example.com, the email might be returned to bounce+123abc+user=domain.com@example.com.