You can only post in a different region's RMB if your region has an embassy with another region. The other region also has to allow residents of embassy regions to post in their RMB.
you can only post on RMBs of your region and regions that your region has embassies with. for example, my region (Antarctica) has embassies with Forest, so i can post on the Forest RMB, but we do not have embassies with CCD, so i cannot post on the CCD RMB. your region has embassies with: Lorania House of Commons, Old Zealand, Union of Allied States, Novus Lucidum, Nordic Lands, The Global Democratic Union, Ciede, Roman Byzantine Union, The Wolf Clan, The Democratic Republic, New World Union, The NewsStand, Zero Zero Zero Zero, The United Ascendancy, The Embassy, The Monarchy alliance, Thaecia, Lorania Courthouse, The United Empires of Carson, The Sportsbook, Morris and Bailey, and Greater Middle East
therefore, you can post on the RMBs of your region and any of those regions, but no other RMBs.
Thanks Cekoviu and Outer Sparta! So basically, for me to post on a region's RMB the region should have embassies with my region and the region should allow posts from other regions. Thanks again.