I have done something similar to this. Here's how I did.

1.) For each loop through your table sending you e-mails. Storing the EmailID you are working with in a user variable (i.e. User::EmailID).

2.) Execute your Send Mail Task

3.) Execute SQL Task

In your Execute SQL Task set the SQLStatment via the expressions tab.

Your expression would look something like this. Note I put single quotes around the variable and the date.

"UPDATE dbo.table SET DatSent = '" + CSTR(GETDATE()) + "' WHERE EmailID = '" + User::EmailID + "'"

There multiple ways to do this I just found this to be easier.

Hope this helps,

Eric