Things we have seen with SYM, are domains using both DNS SPF TXT and the alternative SPF resources records, as the use of the alternative SPF rr is no longer used. RFC 7208 3.1. (superseding RFC 4408) now states that SPF be in DNS TXT only. Having domains drop the alternative SPF rr has helped. But the best solution we have seen helping, is sending in plain text and not in HTML and also dropping signatures in HTML formatting. Albeit not conclusive as it being the direct problem, it has helped many clients.