Count Number of To Addresses (Addressees) in Sieve
Sieve is a mail filter language.
From my understanding of https://tools.ietf.org/html/rfc5231#section-4.2
require ["fileinto", "relational", "comparator-i;ascii-numeric"];
if
allof(
address :matches "From" "test@example.com",
address :count "ge" :comparator "i;ascii-numeric" ["to", "cc"] ["5"]
)
{
fileinto "\\Trash";
}
This filter will move the message to trash if it has >=5 addressees and comes from test@example.com