Kevin, I agree with your comment that the clock stops when the contribution is deposited, but I do not understand you distinction regarding "credited to the account", unless you mean "allocated to to specific participants".
The preamble to the regs say "[deposits]...will be treated as having been made to the plan in accordance with the general rule...when contributions are deposited with the plan...without regard to whether the contributed amounts have been allocated to specific participants or investments of such participants."
Reg 2510.3-102(a)(2)(i) also says: "...any amount deposited with such plan not later than the 7th business day..."
I would struggle with interpreting those underlined sections to mean anything but deposited to the trust, and not to be arguementative, but I certainly don't think they meant when the check is in the mail. I also am not aware of a footnote to the regulation above regarding a mailbox rule.