has recently started resending emails that have been deferred by greylisting from a different IP address that they were originally sent from.  

Because of this, mails can take a long time to deliver before 

the "greylisting triplet" (Sending IP address, sending email address, and recipient email address) matches.

To fix, you add the following IP address ranges to System Setup > Mail Relay > Greylisting > Client IP Exemptions:

To force Greylisting to restart, click the Save at the top of the page.