John Gill Commentary Nehemiah 6:17

"Moreover in those days the nobles of Judah sent many letters unto Tobiah, and [the letters] of Tobiah came unto them." — Nehemiah 6:17 (ASV)

Moreover, in those days
While the wall was building:

the nobles of Judah sent many letters to Tobiah, and the letters of
Tobiah came unto them ;
letters passed between them frequently, they informing him how things went on at Jerusalem, and he advising them to what was detrimental to the true interest of their nation; such false friends had Nehemiah about him, and yet the work succeeded under him; which showed it the more to be of God.