Inconsistent mail delivery issues with Outlook/Exchange

My manager recently returned from a sales convention and noticed that that he's been having trouble receiving messages from certain email addresses. He normally receives a lot of email traffic, but today he's receiving little to none. We suspect that it's an issue with the Outlook or Exchange service itself as he requested an employee to send me an email, but I never received it. Other employees are experiencing the same problem as well. I also went to the Exchange Admin panel to check the Mail Flow, and I noticed several emails from that same employee to different recipients stuck in a loop of being deferred and pending with this event being responsible "Connection refused [Message=Socket error code 10061]," which suggests the issue is with a blocked port or something similar. The problem is inconsistent as some emails both internal and external are able to get through but others aren't, and I can't find anything in common with the emails that can't get through. I checked and we have no rules or connectors in place that could be blocking mail delivery. Our email is hosted entirely on Office 365. I am grateful for any assistance or guidance that can be offered on this issue.