In Eastern Orthodox Churches, we find a difference between the Roman Catholic ideal that Mary was immaculatly concieved (i.e. without sin) as Orthodox Christians reject the western doctrine of original sin, preferring instead to speak of a tendency towards sin. Orthodox Christians believe Mary was born without sin, but so is everyone else. Generally Orthodox Christians believe that due to her faith and action, and the the grace of God, she did not commit an actual sin during her lifetime. She is held as the ideal that mankind can choose not to send and make the right decisions unlike Eve (and ourseleves) who made many wrong decisions and commit sins.
Here are some quotes from the early Church Fathers who attest to this teaching:
"This Virgin Mother of the Only-begotten of God, is called Mary, worthy of God, immaculate of the immaculate, one of the one." Origen,Homily 1(A.D. 244),in ULL,94
Mary, a Virgin not only undefiled but a Virgin whom grace has made inviolate, free of every stain of sin." Saint Ambrose,Sermon 22:30(A.D. 388),in JUR,II:166
"We must except the Holy Virgin Mary, concerning whom I wish to raise no question when it touches the subject of sins, out of honour to the Lord; for from Him we know what abundance of grace for overcoming sin in every particular was conferred upon her who had the merit to conceive and bear Him who undoubtedly had no sin." St. Augustine,Nature and Grace,42(A.D.415),in NPNF1,V:135
"A virgin, innocent, spotless, free of all defect, untouched, unsullied, holy in soul and body, like a lily sprouting among thorns." Holy Martyr Theodotus of Ancrya,Homily VI:11(ante A.D. 446),in THEO,339
“Today humanity, in all the radiance of her immaculate nobility, receives its ancient beauty. The shame of sin had darkened the splendor and attraction of human nature; but when the Mother of the Fair One par excellence is born, this nature again regains in her person its ancient privileges and is fashioned according to a perfect model truly worthy of God…. The reform of our nature begins today, and the aged world, subjected to a wholly divine transformation, receives the first fruits of the second creation.” – Andrew of Crete (733)
"O most blessed loins of Joachim from which came forth a spotless seed! O glorious womb of Anne in which a most holy offspring grew." St John of Damascus,Homily I in Nativ.(ante A.D. 749),in THEO,200
Archpriest Thomas Hopko in his OCA Catechism from the Rainbow Series The Faith notes that:
"We believe that the Virgin Mary is the Mother of God. Through her He Who was God before the ages took on our human nature without relinquishing His divine nature or confusing or "meshing" it with the human nature in any way. The Mother of God is often referred to as the "New Eve," for she said "yes" to God whereas the first Eve said "no." We believe that the Mother of God was sinless of her own free will, that she remains ever-virgin, and that she is the "living tabernacle" of God inasmuch as her womb, as one hymn states, becomes "more spacious than the heavens" by carrying within it the Savior of the world, Jesus Christ."
Bishop KALLISTOS (Ware) in his book The Orthodox Church wites:
"Most orthodox theologians reject the idea of ‘original guilt,’ put forward by Augustine and still accepted (albeit in a mitigated form) by the Roman Catholic Church. Men (Orthodox usually teach) automatically inherit Adam’s corruption and mortality, but not his guilt: they are only guilty in so far as by their own free choice they imitate Adam."
In summary Most Orthodox Christians believe that that due to her faith and action, and the the grace of God, Mary as the Theotokos did not commit an actual sin during her lifetime and was sinless.