Microsoft employees are concerned that the company has been blocking Outlook emails containing the words "Palestine," "Gaza," "genocide," "apartheid" and "IOF off Azure," even if they're including those terms in an HR complaint, according to screenshots, recordings and documents viewed by CNBC.

Employees said they started noticing the change Wednesday just before noon PST, batch-testing emails with the terms in question and emails without them. Only the ones without such terms appeared in their outboxes, suggesting those containing the terms weren't received, according to materials viewed by CNBC and three sources familiar with the matter.

The people asked not to be named in order to speak freely.

One employee with the word "apartheid" in their email signature, who spoke on condition of anonymity for fear of retaliation, said they sent a typical work-related email around 11:30 a.m. PST on Wednesday successfully. The person said that just before noon on the same day, their emails wouldn't go through — ostensibly due to their email signature.

On internal message boards, messages seen by CNBC showed employees asking why their emails with the word "Israel" may go through but not the word "Palestine," as well as "Gaza" and other terms. Modifications like "P4lestine" did go through, according to their tests.

One employee asked on an internal message board, "Is the company abandoning the inclusivity initiative or is this only targeting Palestinians and their allies?"

The Verge was first to report on the potential email block.

In a message seen by CNBC, Frank Shaw, Microsoft's chief communications officer, responded to an employee post, writing: "To clarify, emails are not being blocked or censored, unless they are being sent to large numbers of random distribution groups. There can be a small delay and the team is working to make that as short as possible."

"Over the past couple of days, a number of emails have been sent to tens of thousands of employees across the company and we have taken measures to try and reduce those emails to those that have not opted in," a Microsoft spokesperson said in a statement.

But employees told CNBC that even when they attempted to send relatively mundane, solely work-related emails to small groups of colleagues, the emails still didn't go through if they contained those terms.

Another employee who spoke on condition of anonymity said that when they attempted to send a report to HR containing one of the terms in question, they did not receive the auto-response typically confirming receipt until more than 24 hours later. The message also didn't show up in the online HR portal until more than 24 hours later.

Some emails were delivered after being delayed by seven hours or more, according to the group No Azure for Apartheid. The group suggested manual reviews of such emails were taking place before they were delivered.

Microsoft has seen a growing number of protests at recent events over the Israeli military's use of the company's AI products. Protesters have also sent emails to the company's executives outlining their concerns.

At Microsoft's Build developer conference in Seattle this week, protesters interrupted executives during keynote speeches and sessions.

On Tuesday, protesters interrupted the Microsoft Build session on best AI security practices, singling out Sarah Bird, Microsoft's head of responsible AI, who was co-hosting the session with Microsoft AI security chief Neta Haiby.

Haiby was formerly a member of the Israel Defense Forces, according to a Tumblr page viewed by CNBC.

"Sarah Bird, you are whitewashing the crimes of Microsoft in Palestine," Hossam Nasr, an organizer with the group No Azure for Apartheid, said.

Nasr was one of the Microsoft employees terminated last year after planning a vigil for Palestinians killed in Gaza.

Earlier on Tuesday during another Microsoft Build session, an unnamed Palestinian tech worker disrupted a speech by Jay Parikh, Microsoft's head of CoreAI.

"Jay, you are complicit in the genocide in Gaza," the tech worker, who did not wish to share their name for fear of retaliation, said. "My people are suffering because of you. How dare you. How dare you talk about AI when my people are suffering. Cut ties with Israel."

The worker then called to "free Palestine" and said, "No Azure for apartheid," a nod to the group and its petition.

On Monday, Microsoft software engineer Joe Lopez interrupted CEO Satya Nadella's keynote speech onstage, saying, "Satya, how about you show them how Microsoft is killing Palestinians? How about you show them how Israeli war crimes are powered by Azure?"

Lopez was later fired, according to a document viewed by CNBC that stated the reason as, "misconduct resulting in the violation of both company policy and our expectations of a respectful workplace."

The document said Lopez would be ineligible to return to Microsoft as an employee, contractor, or in any other capacity, including an employee of a Microsoft partner, customer or other third party.

At Microsoft's 50th anniversary event last month, two Microsoft software engineers publicly protested the use of the company's AI by the Israeli military during executive presentations. The roles of both employees, Ibtihal Aboussad and Vaniya Agrawal, were terminated soon after, according to documents viewed by CNBC.