Lemon sat down with CNN Worldwide CEO, Chris Licht, and the two had a "frank and meaningful conversation," according to an internal email Licht sent to CNN staffers on Monday. During their chat, Lemon agreed to participate in formal training, but Licht didn't specify what type.

Licht continued, "It is important to me that CNN balances accountability with a fostering culture in which people can own, learn and grow from their mistakes. To that end, Don will return to 'CNN This Morning' on Wednesday."

Sources with direct knowledge tell TMZ ... Don has not been happy for awhile at CNN, and that's been clear to management and others.  As one source put it, "He's not a happy camper, so who knows how long he stays?"

As you know, Lemon put his foot in his mouth last Thursday when he made sexist comments on air about 2024 Republican presidential candidate Nikki Haley, calling her past her prime at 51. Don was reacting to Haley's earlier remarks that politicians who are over 75 should get mental competency tests.

Lemon said, "Nikki Haley isn't in her prime. Sorry. A woman is considered to be in her prime in her 20s and 30s, and maybe 40s." "That's not according to me" "Look it up. if you Google 'when is a woman in her prime' it'll say 20s, 30s, and 40s."

His female cohosts, Poppy Harlow and Kaitlan Collins, looked stunned and offended. Harlow asked Lemon to clarify if he meant prime for childbearing.  Don nervously responded, "Don't shoot the messenger. I'm just saying what the facts are. Google it."

Lemon didn't stop there, stating, "Nikki Haley should be careful about saying that politicians are not in their prime ... and they need to be in their prime when they serve because she wouldn't be in her prime according to... Google or whatever it is."

Things quickly went downhill for Lemon. Don made several apologies -- including one where he called into a morning staff meeting -- but they weren't well received.