Short answer. An affidavit of electronic filing, accompanied either by a paper copy of the document transmitted or by the clerk of court's written or stamped acknowledgment of its filing. If the paper copy was sent by registered mail, the registered-mail method of proof applies instead.
What the law says
(d) If the pleading or any other court submission was filed by electronic mail, the same shall be proven by an affidavit of electronic filing of the filing party accompanied by a paper copy of the pleading or other document transmitted or a written or stamped acknowledgment of its filing by the clerk of court.
Rule 13, Section 16 — Proof of filing. Read the full provision →
The rule only bites when the document is missing
The general proposition comes first: the filing of a pleading or any other court submission is proved by its existence in the record of the case. The lettered paragraphs address the situation where the document is not in the record but is claimed to have been filed. So if the pleading is on the record, no affidavit is called for — this machinery exists for when it is not.
The affidavit of electronic filing
For filing by electronic mail, the core document is an affidavit of electronic filing of the filing party. The rule then requires it to be accompanied by one of two things: a paper copy of the pleading or other document transmitted, or a written or stamped acknowledgment of its filing by the clerk of court. Either satisfies the requirement, and an acknowledgment from the clerk is generally the stronger of the two because it comes from the court's side.
The cross-reference for paper copies sent by mail
The paragraph closes with a provision that is easy to miss. If the paper copy sent by electronic mail was filed by registered mail, the paragraph on registered mail applies. That means the registry receipt together with the affidavit of the person who mailed it — an affidavit which must contain a full statement of the date and place of deposit in a sealed envelope addressed to the court, with postage fully prepaid, and instructions to the postmaster to return it to the sender after ten calendar days if not delivered.
Other authorised electronic means
Electronic mail is not the only electronic route the rule recognises. Where a submission was filed through other authorized electronic means, it is proven by an affidavit of electronic filing of the filing party accompanied by a copy of the electronic acknowledgment of its filing by the court. The pattern across every mode is the same: the filer's sworn account of what was sent, paired with something generated outside the filer's own control.