Which condition might cause an email to "soft bounce"?

A "soft bounce" occurs when an email is temporarily undeliverable. This can happen for several reasons, but one common cause is that the recipient's inbox is full. When the inbox is full, the email server cannot accept any new messages until space is cleared, resulting in a temporary rejection of the email. Unlike a "hard bounce," which indicates a permanent delivery failure due to issues like an invalid email address or domain, a soft bounce suggests that the email address is valid, but there is a temporary issue.

The other options represent different scenarios that would lead to different outcomes. For instance, not providing a recipient email address would typically prevent the email from being sent at all, leading to an immediate failure. If the domain does not exist, this would also result in a hard bounce since the email server cannot find a route to deliver the message. Likewise, if an email address has been blacklisted, it usually indicates a permanent block, which would not categorize it as a soft bounce, as "blacklisted" means the server won't attempt delivery at all. Understanding these distinctions helps clarify how email deliverability works and the types of errors that can occur.