Cyfeiriad dychwelyd mail receipt section
I think you're asking for the return address for a mail receipt section!
In Welsh, the phrase you're looking for is:
"Cyfeiriad dychwelyd: [insert address here]"
This translates to "Return address: [insert address here]".
If you're looking for a specific format for a return address, you can use the following:
"Cyfeiriad dychwelyd: [Your Name] [Your Street Address] [Your Town/City] [Your Postcode] [Your Country]"
Replace the brackets with your actual information, of course!