A 550 5.7.0 message size violation means a mail server on the way refused your message because it was larger than that server's size limit. Send a smaller message, or share big files as links. The address is fine, so the same person can still get the message.
It is a size problem filed under a security code, which is why it reads like a block. The standard has its own code for a message that is too big, x.3.4 in RFC 3463, and we would rather servers used it.
How to read the result
Paste the whole bounce. The words "Message Size Violation" pick the size meaning, because other bounces share the code: Gmail uses 5.7.0 for content it treats as a security risk and for a refused Workspace relay. If your text says nothing about size, the fix is different.
Then look for the server. A "Remote Server returned" line names the one that refused, and the limit was its own. No remote server usually means your own outgoing server or relay stopped the message before it left.
Why it was bigger than the file
Mail carries attachments as text. The base64 encoding in the MIME standard writes every three bytes of a file as four characters, then adds line breaks, so a file under a limit can arrive over it.
Encryption usually encodes the message again for the trip. An attachment that fits in the clear can cross the limit once encrypted, which is why the encrypted copy bounces while the plain one went through.
What to do next
If the limit is on your own server, raising it is a setting. Otherwise, send a smaller message or a link to the same person.
In a first cold email, we would send a link and no attachment anyway: it does the same job at a fraction of the weight.