Anti-virus software can quarantine suspect files when they are created. This can have undesirable effects for email decoding, such as when file attachments are decoded and streamed to disk.
After the file is quarantined, any attempt to access the file will cause an exception to be thrown (such as a FileNotFoundException or UnauthorizedAccessException). This exception can be caught, and an indication given to the user that the attachment may have been removed by third-party software. If access to the file is desired regardless of its content, setting Attachment.DecodeToMemory to true may allow you to circumvent the anti-virus. Attachments will be stored in memory (instead of on disk) and can be accessed using Attachment.GetContentStream().