ejay69
...as I have seen mail programs and the webmail maintains user@parentdomain as senders address.
That is correct for webmail, as webmail is an application running on the server and does get default information about the "main" domain.
Not so for email clients. Every email client I have ever setup requires manual input of the senders/return email address.
You or the end user at some stage must configure the email client manually, and it is at that stage you only tell them about using the domain name that you require them to use.
As far as the centralised webmail is concerned, then you will need to login as each user unitially and configure their defaults for the domain you require them to appear to be sending from. This is explained in the documentation you were given links to.
Which part of the above do you not understand ?
The issue as I see it, is how users set up their email clients, not how sme server is setup (apart from issues with webmail client defaults).
If you have control over the end users workstations, then you can tailor the Default User profile to dish out the correct default settings in your email client of choice. Your situation does not sound like you have that degree of control though.
The other "radical" workaround answer, is to only host one domain on one sme server, and that was also mentioned in the documentation.