Probably means your email address was used as the sender for some spam. That’s not an indication of any issue with your mail or your security, just that some spambots faked the sending email address and happened to use your email address. Ignore it, annoying as the “bouncing” messages will be until the spam run finishes.
There are also some email scams that fake these messages, but those too can be ignored.
You’ve specified macOS Server as your footer here, and I am assuming you are not using that product and not running your own mail server. Older versions of macOS Server included a mail server, and it is possible to misconfigure that and to get into some trouble.
If you are on a Mac, you can add a message filtering rule to process mail bounces.