Depends on what's blocking those NDR(s). Do you have any supporting logs/tracking as an insight. One of the behaviours thats known to us is with Auto reply from Microsoft/O365 when bounce verification is enabled.
For e.g. when an email sent from ram@xyz.com" processed by an ESA which bounce verification enabled results in a prvs tag addition to envelope sender. When the recipient is out of office, the exchange server doesn't respond to the "return-path" address (which has the prvs tag) rather uses "from" address. Since these OOO originates from null/empty senders, ESA classifies them as bounce, drops them since the recipient address doesn't contain the prvs tag.
