Could you possibly elaborate a bit more on the public folder first email to be created.
This mailbox must contain a valid email address which contains a domain that works with autodiscover for all customers. (You may need to use powershell to set this email address)
Not sure I understand it when you say a domain that works with autodiscover for all customers. If you enter first folder, it will automatically create an email called firstfolder@yourrootdomain.
Your first public folder mailbox is considered the master public folder mailbox. The PrimarySmtpAddress (email) must be using a domain that has a working autodiscover. So if your mail server is hoster.com then you can make it email@example.com if autodiscover works for that domain.
By default it will point everyone at the master public folder mailbox (in this example would be firstname.lastname@example.org), but if they have their own public folder mailbox then it will point to their own which should have a unique email address that uses their domain name.
If you use a domain on either one that doesn't have autodiscover then they will get prompts / it won't work.