One of our prominent members poo poo'd that with the reasoning that you wouldn't be able to prove what was in the letter you sent.
Fwiw, whenever I send something important enough to send certified mail, I complete the USPS form and then photocopy the letter with the form so both are 'connected'. Further, since I keep a stack of these forms in my office, I generally include a reference to the item number in the letter text. Something like:
Via USPS Certified Mail Item # xxxx xxxx xxxx xxxx
Once mailed, staple the certified receipt to the photocopy of the letter and I believe you've established not only the letter contents but also the date of mailing.
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Shamr0ck