Indiana Code Title 6. Taxation § 6-8.1-6-3

Sec. 3. (a) A document, including a form, a return, a payment, or a writing of any type, which must be filed with the department by a prescribed date, is considered filed:

(b) If a document is sent through the United States mail by registered mail, certified mail, or certificate of mailing, then the date of the registration, certification, or certificate, as evidenced by any record authenticated by the United States Post Office, is considered the postmark date.

(c) If a document is mailed to the department through the United States mail and is physically received after the appropriate due date without a legible correct postmark, the person who mailed the document will be considered to have filed the document on or before the due date if the person can show by reasonable evidence to the department that the document was deposited in the United States mail on or before the due date.

(d) If a document is mailed to, but not received by, the department, the person who mailed the document will be considered to have filed the document on or before the due date if the person can show by reasonable evidence to the department that the document was deposited in the United States mail on or before the due date and if the person files with the department a duplicate document within thirty (30) days after the date the department sends notice that the document was not received.

(e) If a document is received after the deadline, the department shall consider the document to be received on time if the postmark date is up to three (3) business days (not including a day falling on Saturday, Sunday, a national legal holiday recognized by the federal government, or a statewide holiday) after the date of the deadline.