Answer
(Fatwa: 1543/B=314/TB=1432)
Neither the voice is recognized by email that who is making wakil to whom nor the signature is known. Hence it is not right to make someone wakil for one’s nikah or transfer the power by email. Yes, one can make some particular person wakil by phone or writing. For example the girl allows her nikah by writing or phone and makes someone wakil that her nikah may be done with so and so person. And if the nikah was performed in the presence of two witnesses as per Shariah then nikah shall be valid. But, such nikah sometimes creates problem; hence one should avoid doing nikah by phone.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband