Emails sent from Post Affiliate Pro are delivered only to some email servers, but not to all and are often marked as apam.
Check your email (SMTP) server logs, if you see log similar to this:
May 9 13:01:00 example postfix/smtp: 71F4110179: to=<firstname.lastname@example.org>, relay=qualityunit.com[188.8.131.52]:25, delay=2.3, delays=0.53/0.01/1.6/0.15, dsn=5.0.0, status=bounced (host qualityunit.com[184.108.40.206] said: 550 Access denied - Invalid HELO name (See RFC2821 220.127.116.11) (in reply to MAIL FROM command))
probably you have wrong configured host name for your email server.
Check if it contains the fully-qualified domain name of the SMTP client. In situations in which the SMTP client system does not have a meaningful domain name, the client SHOULD send an address literal.