David, per you last comments above when the client sends ""exact same email manually"" how are they doing this? Are they using PL again via the Viewer resend function, or are they using their local email client to send from personal account as attachment? Leading to... is the sender the same?
It is either with the sender or somehow the bulk operation is being flagged as such. The good news is it doesn't seem to be a domain level blacklist issue. Since mentioning customers, I am assuming we're talking about AR and / or SO invoices / orders, correct? If so, then another assumption is we're talking about multiple customers with various filtering systems which makes it more complicated to figure out cause. In case of DD stubs, have seen bulk get flagged which dictates sender needing to be added as approved bulk sender because so many are being sent to same domain (usually internal). That doesn't seem to be the issue here. Always a puzzle.