Because you’re saying that this is an issue when sending to more than one person or sending more than one file, it sounds like it could possibly be a restriction on how much information that your Outlook client is configured to send. I think that they use to limit it to 2MB by default, see if you can find where that restriction is and increase it. Otherwise another thing that could possibly be causing the attachment problem is an antivirus program scanning the email before it leaves your computer and thinking it’s infected or corrupted so it stops it at the door. See if your antivirus has a setting to stop looking at outgoing emails and see if that makes any difference.
I hope this helps