The automatic sample submission feature that is part of Sophos Online Scanning allows samples of malware and suspicious files to be sent to Sophos Labs for analysis, and considerably speeds up the process. However, if clients use a security proxy with AV features to access the Internet, although the data uploaded is encrypted, it's possible that the upload will be blocked as suspicious.
In order to avoid the upload being blocked as suspicious, you must ensure that access to samples.sophosxl.net is not blocked by the proxy.
The way in which you achieve this will depend upon on the type of software or device used.
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