When a email has bounced there will be the following explanations that is provided by the recipients mail server:
General: The recipient's email provider sent a general transient bounce. This typically happens when the recipient has set up an "out of the office" automatic reply. This type of soft bounce generally fixes itself. If the bounce was a result of an "out of the office" reply, then the soft bounce resolves itself after the automatic reply is removed by the recipient.
MailboxFull: The recipient mailbox is full and doesn't have the capacity to accept any more emails. You might be able to send emails to the same recipient in the future when their mailbox is no longer full.
MessageTooLarge: The recipient mail server can't accept the email because the file size of the message is too large. You can try to resolve this type of bounce by reducing your message size.
ContentRejected: The recipient mail server doesn't allow the content of the email. You can try to resolve this type of bounce by modifying the content of your email.
AttachmentRejected: The email contains an unacceptable attachment. Some email providers block emails with attachments of a certain file type or large attachments. After you remove or modify your email attachments, you can try to send the email to this recipient again.