SPF cause my mail to be rejected
Type their domain in to this tool (an SPF record checker) and see if it passes.
http://mxtoolbox.com/spf.aspx
http://www.kitterman.com/spf/validate.html
The solution that worked for me is adding DNS TXT record.
TYPE NAME(YOUR DOMIAN) VALUE TTL
TXT sydneysideloaders.com.au "v=spf1 include:spf.protection.outlook.com -all" 3600
If no problem found, the problem is on their end. an invalid SPF record means it
could be spam / or a forged address and it seems reasonable to me to reject such
messages.
TLDR It's their problem, tell them to fix their SPF record. The alternative
makes YOU less secure.