The easiest way I found was to use mutt to send the files directly to mx1.sophos.com.
Step 1: create a muttrc file with only two lines:
set from = 'XXXXX@XYXYXY.me'
set smtp_url = 'smtp://mx1.sophos.com:25'
Step 2: use this mutrc file to send virus sample:
mutt -F sophos-mutrc samples@sophos.com -s "Virus sample I found" -a firus_file.zip
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