[sheikheddy]

This is correct but not the only reason it could be happening.

[currysausage]

But why do you consider this good practice? It's (unnecessarily?) frustrating for senders and poses a legal risk for recipients (the sender has the logs to prove that they sent the invoice, while the recipient doesn't have any record).

[Avamander]

Again, not the person you replied to. But some feedback mechanisms take time (so action has to be taken after a 2xx reply) and some indicators are just very very very accurate that leaving them in even just Spam is a way bigger risk. Users have a terrible tendency to dig out malware from Spam folders.