Some good Christian closing salutations for emails are "In Christian love," "Because of Calvary" and "In His Grip". These salutations are recommended by David Peach, who has experience in full-time missionary work.
Peach also recommends some Bible verses for closing an email. His recommendations are "Grace be unto you, and peace, from God our Father, and from the Lord Jesus Christ" (1 Corinthians 1:3) and "Whether therefore ye eat, or drink, or whatsoever ye do, do all to the glory of God (1 Corinthians 10:31).
"Your servant in Christ" is one of the Christian closing salutations in Susan Adams' list. Adams, a Forbes Senior Editor, does not recommend using a Christian closing salutation in an email to a non-Christian.