If you are using the "free" 3CX SMTP servers this is inserted by those systems. To get around it you will need a Pro or Ent license key and your own SMTP server so you have more control of the sending email address. Changing the sending "name" is still difficult, as I don't believe this is supported in the default templates.
You might be able to hack it a couple different ways on the recipient side. For example with Office 365 if you make an Email contact and add it to the Global Address Book I believe it will use the address book display name.
@accentlogic
I'm using a pro license and even when sending via gmail's smtp this shows up. I don't believe this is inserted by their mta, but inserted by the PBX as a result.
@cobaltit
I was afraid I'd have to do some stmp/mta trickery to scrape that out.