Md. Code, Election Law § 9–505

(a) A vote–by–mail ballot is considered timely and may be counted if the ballot:

(1) is returned in person to the office of a local board by the voter or the voter’s duly authorized agent no later than 8 p.m. on the day of a special election; or

(2) (i) is received by mail by a local board no later than 10 a.m. on the second Friday after a special election; and

(ii) was mailed on or before election day, as verified:

1. by a postmark; or

2. if the return envelope does not contain a postmark or the postmark is illegible, by the voter’s affidavit that the ballot was mailed on or before election day.

(b) A vote–by–mail ballot that does not meet the requirements of subsection (a) of this section is not timely and may not be counted.