We have been asked to submit our letter to Po Box 123.

[tufguy]

1) We have been asked to submit our letter to Po Box 123.

2) We have been asked to submit our letter at Po Box 123.

Do we need to use "To" or "At" in these sentences?

 

[GoesStation]

"To". You'd normally send a letter to a P.O. Box.

 

[Rover_KE]

Or, in British English, 'to a PO Box'.