Code
T0021 mail-not-sent
The mail server did not take a message. The log holds its answer.
Details
When the server took the message for some addresses and not for others, the message went to those it took, and the error names the rest. A second try of the same call sends the whole message again, to those who had it too: code that minds catches this code and sends to the rest.
The fix
Read the answer of the server in the log, with tent logs. Where the error names addresses, send the message again to those.