How do I return to SMS text messages in Marshmallow?

[AC Question]

I have a Samsung Galaxy S6 (base model, not Edge, Plus, or Active) on T-Mobile and we finally got the Marshmallow update recently. It has totally and completely messed up my texting (and my keyboard, but that's another issue). Some of my messages are treated as chats. Every time I start typing I get a message that IMS has stopped working. It sends some of my messages as MMS even though they're not long and have no attachments. When I send a message to more than one person, it sends it as a group MMS and replies go to the whole group. (This was a feature I have long hated because we send a lot of group texts through my workplace. I was thrilled when I first got the phone and discovered Lollipop was sending group messages as SMS and that replies only went to the sender, not the whole group.) I don't get notifications for these non-SMS texts on my Gear Fit.

Short of resetting the phone back to Lollipop (which I'm tempted to do), is there any way to fix my text messaging? I already have chat turned off. The only messaging app I have installed is the default one, so I don't have the option to switch to a different messaging client. I've uninstalled/disabled Hangouts, WhatsApp, Messenger, Facebook, any type of social media, and I've never had an alternative texting app like Snapchat or Go SMS Pro installed on this phone.

 

[Rukbat]

MMS triggers when the message is longer than 140 octets (which is 80 characters if the alphabet you're using is unicode or 160 if it's ASCII). It also triggers on any non-alphanumeric character - like an emoji, and any multi-recipient message.

 

[virtuouskate]

MMS triggers when the message is longer than 140 octets (which is 80 characters if the alphabet you're using is unicode or 160 if it's ASCII). It also triggers on any non-alphanumeric character - like an emoji, and any multi-recipient message.

Thanks for your response. I'm the original poster. I know how MMS is *supposed* to trigger, but I'm getting the pop-up screen about the IMS error even when I'm not composing a long message or using a non-alphanumeric character. Back when my phone was running Lollipop, it sent group texts as MMS, yes, but when someone replied, their response went only to me, not the whole group, and no one had trouble opening my texts. And the chat thing with some people is driving me nuts. I just all-around want to return my texting back to the way it was.

I hate pretty much everything about Marshmallow and I'm thinking about doing a factory reset, but I know there are always security concerns when you're running an outdated operating system, so first I'm trying to find solutions to the things I most dislike about the new system and learn to live with it.

Thanks again.