As we all know, Spam is prevalent on the internet. Google is fighting a seemingly never-ending battle against spammers. Spammers change their tactics and Google changes it's anti-spam countermeasures in response. Unfortunately sometimes legitimate posts and emails get inadertently caught by the spam filters.
One Help Forum user recently (April, 2024) ran some tests to compare success rates of different methods of posting to a Google Group.
See this thread https://support.google.com/groups/thread/266401203?hl=en&msgid=268689772 the contents of which I have reproduced below. (Of course, now that this information has been published, spammers may try to take advantage of it and Google may adjust their spam filters accordingly so there's no guarantee that the results found by the poster, Jeremy Moskowitz, will remain valid 😐
"Okay. I did some testing today to figure out what generated the MOST SUCCESS to LEAST SUCESS to get emails / posts out the door... and not caught / trapped for moderation.
BEST: Going right to groups.google.com/g/YOURGROUP then starting a New Conversation. These succeeded 100% of the time. Everything worked, including complex emails with links and formatting.
SECOND BEST: Using your email / Outlook / Gmail / Yahoo / Whatever... and setting the message to PLAIN TEXT. Even links were evaluated and HTML-ified when the recipient views them; which is nice.
Near guarantee of being trapped: Using your email / Outlook / Gmail / Yahoo / Whatever... and sending the message as HTML.
Almost completely sure to be trapped: Using your email / Outlook / Gmail / Yahoo / Whatever... and setting the message to HTML and having lots of links or ANY attachments.
This was my experience in my testing, your experience may vary. I hope it helps you."