I don't follow.
What you need is facts, not generalities such as we're having issues with mail. Remember, TEC did receive your WS, so the question which remains is WHY did they reject it. Was this?
It was completed incorrectly;
It was received late;
Or both?
We're still blind.
Your WS was dated X
They received it into their system on Y
They rejected, but we don't know why.
For obvious reasons TEC will only act on when they received the WS, NOT the date someone wrote on it.
So: you need date of posting - sadly you'll have to rely upon RM as you've mislaid your proof of posting.
You also need the date TEC received it - for completeness.
Was the WS completed correctly- as you didn't keep a copy you'll have to rely upon TEC.