Greylisting is a primary means of eliminating spam. This simply means that the FIRST TIME a server connects to try and send you email, it is told to come back later. After 5 minutes (which is not very long) an email from that server will go through. Most spammers will not go through the trouble of attempting re-delivery, and therefore a large amount of junk email (currently over 60%!) can be eliminated just by using greylisting.
To re-state, your email server will only issue a 5 minute delay to the FIRST EMAIL delivered from a "new" server. After that, the sending server is allowed to connect and send immediately for 7 days. And every time that sending server re-connects, the 7 day clock is reset. Therefore, it's possible that the issue will work itself out automatically and after the first few connections will no longer be a noticeable issue at all.
If that is an unacceptable solution, we have 2 suggestions for you, and you can choose what you prefer:
#1: Disable greylisting for your email. We can certainly disable the greylisting, and that will resolve the issue from this sender. However, you are likely to see a marked increase in the amount of spam you receive as a result. So that is a trade-off that you will want to weigh carefully.
#2: Encourage your sender to utilize best practices to ensure that their systems are compatible with modern spam-fighting techniques. In most cases, this is easily achievable and knowledgeable system administrators can accomplish this within a very short period of time without having to perform extensive work. The most frequent extended delivery delays are caused by email systems that attempt re-delivery on deferred messages via a different system. This further delays the message, since the new IP address must then attempt a redelivery after 5 minutes. Due to this misconfiguration, an email that would naturally come in after 5 minutes may take significantly longer.