Is it valid to reconcile with a wife who was divorced via email by saying: "Go, and you are divorced," and what are the conditions for this reconciliation, and is it conditional upon witnesses, or is it valid over the phone?

If this is the first or second divorce, it is permissible for the husband to take back his wife before the expiration of her 'iddah (waiting period). This period is three menstrual cycles for women who menstruate, upon delivery for a pregnant woman, and three months for women who have despaired of menstruation. The return (raj'ah) is effected simply by saying, "I have taken back my wife," whether over the phone or otherwise. It is not a condition for the wife to be aware of it, nor is it required to have witnesses to the return, though it is recommended.