And to add... the Department of Defense specifically sent out the memo so that retired personnel knew they were still covered under the Emoluments Clause:
"CONCLUSION
The Emoluments Clause to the Constitution applies to all Federal personnel. The Clause prohibits receipt of foreign gifts unless Congress consents such as in the Foreign Gifts and Decorations Act, 5 U.S.C. § 7342. For retired military personnel, the Emoluments Clause continues to apply to them because they are subject to recall. The Justice Department opinions referred to in this paper construe the Emoluments Clause broadly. Specifically, the Justice Department construes the Clause to include not only gifts of travel and food, but also payments such as proportionate profit-sharing. To avoid an Emoluments Clause problem resulting in suspension of retired pay, retired military personnel should seek advance consent through their respective Service consistent with 37 U.S.C. § 908. It is prudent for retired military personnel to obtain advance approval even when there is uncertainty about the Clause’s applicability. Finally, if a retired military member suspects that he or she has violated the Clause, but wants to continue to perform compensated work for a foreign state, he or she should expeditiously seek advance consent for future compensated work, and terminate current compensated employment with the foreign government until such approval is granted. This would be done to avoid increasing the amount of an erroneous payment."
http://www.dod.mil/dodgc/defense_ethics/resource_library/emoluments_clause_applications.pdf
If Congress wants to, they can not only put Flynn in jail...but he would lose all his military retirement pay and benefits.
My opinion, I'm guessing they simply fine him the amount he was paid by Russia and that's it. It will be a compromise between the Republicans and Democrats. That is if Trump doesn't pardon him.