One email gets instantly sent, ... the other gets a 30 minute delay via PHP mail() function

One email gets instantly sent, ... the other gets a 30 minute delay via PHP mail() function

Hi,

Having problem with some automatic emails my system sends to customers.

One email gets sent instantly, the other email gets delayed by 30 minutes, both in same PHP script using same PHP mail() function

mx.zohomail.com; spf=pass (zoho.com: domain of uk3.wpxhosting.com designates 185.5.172.158 as permitted sender) smtp.mailfrom=****@xx.com

One gets permitted, which is the delayed one, and the one that doesn't get permitted is the instant one, like this

mx.zohomail.com; spf=softfail (zoho.com: transitioning domain of _spf.google.com does not designate 185.5.172.158 as permitted sender) smtp.mailfrom=****@xx.com

How do I force the delayed one to also not be the permitted sender? That's the only way I can think of fixing this issue...