Boeing turned the information over on Thursday to the US' Federal Aviation Administration, which said the company had discovered the messages "some months ago", reports Efe news.
The revelations are the latest for the commercial aerospace giant, which was forced to ground the MAX in March due to safety concerns. They also mean more uncertainty for Boeing shareholders, who are keenly focused on when the MAX might return to service.
The 737 MAX has been globally grounded since March, as investigators evaluate the airplane's safety following the fatal crashes in Indonesia and Ethiopia, which claimed the lives of 346 people.
Boeing's stock has fallen 9 per cent since the flying ban, although it is up year to date.
Following the revelation, Boeing's shares fell nearly 7 per cent on Friday to $344.
Boeing will report third-quarter earnings on October 23.
Boeing CEO Dennis Muilenburg is scheduled to testify on October 30 before a House transportation committee on the plane's design and certification, and may appear before a Senate committee, as well.
The instant-message revelations are almost certain to be on the agenda.
Finally, Boeing will submit proposed fixes for the MAX to global aviation authorities later this fall.
Southwest Airlines, which flies an all-737 fleet, has removed the newest model 737 from schedules through February 2020.
United Airlines Holdings recently removed the MAX from its schedules until January.
Investors also want an update about production. Boeing cut back production of the plane in April. Since then, about 275 new jets have rolled off assembly lines and been parked. The company might cut production again to manage inventory levels and cash flow.
More delays and production cuts sound bad, but if delays stretch out just a few more months, the updates could be good news for the shares.
( With inputs from IANS )