I efiled my client's return in February. He paid the IRS and mailed it in on April 15. It was a Cashier's check. The IRS did not cash it until May 8. They are hitting him with interest and penalties. In his mind, since it was a Cashier's check, then the bank had already taken the money out of his account. He wrote the IRS a letter explaining the above. He has no proof of the date he mailed the check to the IRS. I explained that without proof of date of  mailing, the IRS would most likely deny his request to remove interest and penalties. Does anyone have any suggestions or ideas that might help my client?