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 from that can take a while to deliver before you get the "triple match" (Sending IP address, sending email address, and recipient email address)

To get around this, you should add the following IP address ranges to System Setup > Mail Relay > Greylisting > Client IP Exemptions:

==> These IPs were gathered from their spf record