The software company said the fault appeared to be with a third-party provider.
Down Detector, an internet tool, noted that some Etisalat services and Du services were interrupted from about 10.30am and continued for much of the day.
Etisalat said the issue was due to a fault in an underwater cable that it uses.
"Please note that some of the slowness that you encounter while using applications and websites is not related to the performance of the network in the country or its capacity, but rather due to a cut in one of the internet international cables," it said.
On Twitter, a number of users said they were able to access Outlook mailbox by using phones and phone hotspots for their computers.
"We suspect that a third-party DNS issue is the cause of impact," Microsoft's 365 helpdesk said.
"We're looking at network telemetry to isolate the source of impact. As this issue may impact multiple services, updates can be found under the new service incident MO313750."