Extra spaces are inserted into the subject of the message when it is tagged by Anti-Spam or Email Data Protection detection. The issue doesn't occur when the subject contains only alphabet, number and symbols. It occurs if multi byte characters are mixed with them.
Original Title : あなたにABC社より当選のお知らせです
Tag : [SPAM]
Result : [SPAM] あなたに ABC 社より当選のお知らせです (Extra space before and after "ABC")
Expectation : [SPAM] あなたにABC社より当選のお知らせです (No spaces added)
This issue occurs under the condition that the subject contains both single byte and multi byte characters, and its encoding does not conform with MIME specification (RFC 2047).
More precisely, encoded headers need to conform the following description in RFC 2047:
an 'encoded-word' that appears in a header field defined as '*text' MUST be
separated from any adjacent 'encoded-word' or 'text' by 'linear-white-space'.
This is expected behavior compliant with RFC.
Please consult with the message sender to use RFC conformed MUA or messsage generation systems.