Hi,
I have huge experience and troubleshooting mail deliverability issues.
Modern mail systems (like Gmail, Hotmail, etc) to prevent spam these days
require each inbound email to pass number of tests... if it not pass - system decides it's probable spam and declines it.
To understand the real reason of your problem please send exact mail you had problems with
to my email :
mybestexpert at gmail dot com
By analysing mail headers I'l try to discover issues and work out recomendations.
Best,
Alex