Genesis 21:7

And she said, How faithful was the messenger who announced to Abraham, and said, Sarah will nurse children, for she shall bring forth a son in her old age!

‘MI’ (WHO) WOULD HAVE SAID UNTO ABRAHAM. The word mi is used as an exclamation of praise and distinction. The sense of the verse is thus: “See Whom it is and to what extent He keeps His promise. He promises and performs!” Thus the language of Rashi.
But we do not find the word mi used in this way in expressions of distinction and honor. Instead, we find it used only in a derogatory sense: ‘Mi’ (Who is) Abimelech, and ‘mi’ (who is) Shechem? ‘Mi’ (Who is) the son of Jesse?
The correct interpretation appears to me to be that Sarah said, “G-d hath made laughter for me; and every one that heareth will laugh on account of me, filling his mouth with song and laughter for the wonder that has been done to me, for who among the hearers would have previously said to Abraham that Sarah will suckle children? There is not a person in the world who would have told him this, even merely to console him, for the possibility would never have occurred to anyone.” Onkelos’ rendition is close to this interpretation: “How faithful is He who spoke to Abraham! He has fulfilled His word that Sarah will suckle children.” That is to say, “Everyone that heareth will laugh on account of me” for there is no person who would have maintained his credulity even in the eyes of Abraham if he were to have told him this wonder.

ותאמר: מי מלל לאברהם, everyone knows that it was the One and only G’d Who had foretold Avraham that Sarah would bear a son for him.

כי הניקה שרה בנים, such a prophecy that Sarah who had become old without ever conceiving would nurse children could only have emanated from the One and only G’d!

ותאמר, she said something else reflecting her happy surprise,

מי מלל לאברהם, who would have dared foretell Avraham that Sarah would nurse babies? If G’d Himself had not been at work in this, no one would have foretold Avraham that something like this could occur.

לזקוניו, we already explained the meaning of this word in verse 2. As to Sarah speaking of בנים, children in the plural mode, although she had given birth to only one child, she assumed that if she had been able to nurse one child there would be nothing to prevent her from nursing more children if she so desired. There is an allegorical Midrash on this verse (quoted by Rashi) saying that many women did not believe that Yitzchok was drinking mother’s milk from Sarah’s breast. In order to silence their doubts or derision, Sarah offered to nurse also the children of such women.

'ותאמר: מי מלל וגו, she said: “who would have dared predict, etc.” This was a way of giving thanks to the Lord. It is similar to Isaiah 3,10: אמרו צדיק כי טוב, “tell the righteous that he has done something good.” Sarah here gave thanks to whosoever had foretold her husband Avraham that she would bear him a son in her old age and would even be able to nurse not only him but also other babies. In other words: she thanked the Lord. An alternate interpretation of this exclamation of Sarah: “Who would have foretold that Avraham was still virile enough still to have impregnated Sarah so that she could produce a healthy son.” According to this interpretation the word מלל is similar to the word מלילות in Deuteronomy 23,26: “(plucking) ears of corn.”