Quite a few of my pictures which feature totally natural sunsets with not filters have been rejected

I can understand how disappointed you feel with most of your photos rejected. I will assume your reluctance to upload photo for us to see, is as a result of you having difficulty doing so. It would be easier for us to identify the actual issue, however I will try to help the best way I can.

From your comment I am made to understand that you're aware that limited post processing is encouraged. Therefore I will not elaborate on that. In most cases if not all artifacts refers to grain and/or noise in your photos. My experience taught me that even during a series of photo shoot whereby settings remain the same, more grains appear on some than others. (In my case this result in the need to discard some of these photos.) You can identify noise in darker places on your photos. Grains can be identified at smooth places. You however need to zoom your photos to 100% and more to identify some noise or grains - up to 200% is advised.

ISO determine how grainy your photo is. Low ISO gives fast shutter speed and works better in well lit areas such as outdoor during the day. Low light, on the other hand, require slower shutter speed, hence higher ISO. Now with higher ISO more grain is introduced, hence the need to increase the in-camera exposure to compensate for the lowlight; and depending on how dark, wide angle helps to capture more of whatever light available. This will require practice since you can sometimes end-up with over exposure. During a series of outdoor shoots at day, light quality changes sometimes depending on cloudiness. I'm not sure if that was your case during your shoots.  If there's not too much noise you can correct it with you editor. Adobe has a nice compact manual at tagproducts_SG_STOCK-CONTRIBUTOR_i18nKeyHelppagetitle . Tap/click on "Adobe Stock Contributor Guide". Tap/click on the link. A PDF will open. In the content tap/click on the page number for "The Review Process". See what you can glean from "Improve Your Content Game". The following page might also be useful Create better photos for Adobe Stock with 7 tips for success |​ . Depending on your needs you can follow the "Useful Links" at the end of the PDF Guide. I hope this helps.

Best wishes

Hello, We will need a few of the photos to see what the problems might be. Just for fun, you tell us what your guess, based on Adobe Stock Guidelines. These guidelines are worth reading - not just once.  Best regards, JH

For  you to be able to achieve better image results and get offerings accepted into Adobe Stock go to   https://helpx.adobe.com/stock/contributor/user-guide.html