The word temporarily is the key: this is a soft bounce, a 4xx level rejection meaning try again rather than give up and other AOL recipients working narrows it to something specific about this one mailbox or this one sending moment. The causes and what each means for you:

Their mailbox side, most common: the recipient's AOL mailbox is full, over its storage quota, so AOL holds your mail off rather than bouncing it permanently, expecting the box to clear. Your mail server will retry automatically for a day or more, and the message often delivers once they clean up. This is genuinely their problem, and the action is telling the colleague their mailbox is full, not troubleshooting your end.

The AOL side throttling, second: AOL rate limits senders it does not yet trust, and a temporarily unavailable can mean AOL is deferring your mail while it assesses your sending reputation, especially from a new IP or after a volume spike. Other AOL recipients working argues against this, but a new mail server sending its first mail to AOL sometimes sees deferrals that clear as reputation builds. The retry mechanism handles it, and persistent deferral means reputation work, SPF, DKIM, DMARC and AOL's postmaster resources.

The transient genuine case, third: AOL infrastructure having a moment, a real temporary unavailability on their end that clears by itself, the retry delivering when it does. Nothing to do but let the retries run.

The action that fits all three: check whether the mail eventually delivers, since temporarily unavailable mail your server keeps retrying often arrives within hours without intervention. A message still failing after a day of retries escalates, to the colleague about their quota first, then to sending reputation if a pattern emerges across AOL recipients rather than this one.