The certificate of mailing must provide the following for each piece of correspondence submitted:

1. (1) attest to the mailing and the address used;
2. (2) include the name of the document and the application serial or registration number or USPTO reference number, if assigned, to which the document pertains;
3. (3) be signed separately from any signature for the correspondence;
4. (4) be signed by a person who has a reasonable basis to expect the correspondence would be mailed on the date indicated; and
5. (5) state the date of deposit in the mail with the USPS, which must be a date within the set filing period (this includes the last day of the period, or the succeeding day that is not a Saturday, Sunday, or Federal holiday within the District of Columbia when the last day of the period falls on a Saturday, Sunday, or Federal holiday within the District of Columbia).

37 C.F.R. §2.197(a)(2).

The best location for the certificate of mailing is at the beginning of the correspondence to which it pertains, typed in its entirety. The certificate of mailing should be separated from contents of the correspondence that are on the same page. Several blank lines between the contents and the certificate will suffice.

If the certificate of mailing does not fit on the correspondence to which it pertains, the certificate may be placed on a separate sheet of paper that is attached securely to the correspondence. The separate sheet may be a cover letter, with the certificate placed at the bottom of the letter and signed separately from the letter. If there is any doubt concerning the correspondence to which a certificate of mailing on a separate sheet relates, the USPTO will not accept the certificate.

When correspondence for more than one application or registration is mailed in a single envelope, each item of correspondence must have its own certificate of mailing. Similarly, when more than one type of correspondence is submitted in connection with the same application, each item of correspondence must have its own certificate of mailing.

It is suggested that the certificate be signed by the applicant, registrant, or the party involved in the proceeding, or by the attorney for such person. If someone else signs, it should be a responsible person in a position to know that the mail will be deposited on the date specified. See 37 C.F.R. §2.197(a)(2)(iii).

The USPTO accepts the date of deposit stated in the certificate of mailing on the basis of the statement of personal knowledge. The USPTO does not normally inspect the postmark on the envelope.