Why are you converting to array, and then to a list? Also, if your email message supports AddRange, an alternative is to try the following?
var addresses = (from p in emailMessage.EmailParamHeaders.To
where p != null
select p).ToList();
if (addresses != null && addresses.Count>0)
m.To.AddRange(addresses);
Depending what's going on inside the Add method, you can often find that an AddRange works more efficiently.
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