If you use Gmail to pull in email from a 3rd party email server then you will hope that you never see this message. This message happens when Gmail won’t download an email from your mail server due to an attachment contained on the email.
In this case the email was unexpected and from the from email address alone it appears that it’s a valid concern that the attachment may be suspicious. In fact more than likely the attachment was of the infectious nature.
This is one reason to love Gmail, not only do their protect you from mail that goes through their servers but they also will protect you from mail you download through a separate mail server. That is an added level of security that helps set it apart of the rest, not that you should ever download attachments contained in emails from people you don’t know or aren’t expecting.