Emails are being automatically deleted from my inbox

I have an iMac using Macmail. My emails are being automatically deleted from my inbox after a certain period of time (appears to be about every two weeks). This started happening after I changed from a POP server to SMPT. My mail first goes to my internet email account (roadrunner.com - mail.twc.com website - was Time Warner Cable, now owned by Spectrum). I have checked all of the settings on the TWC mail page with a tech support person there and all are ok. I also checked the Macmail settings on my computer and they all are ok. The emails aren't in any of the deleted mail or trash folders on my mac or on twc mail, they are just disappearing altogether. I'm using Firefox as my browser if that makes any difference...

Any help on how to fix this problem would be greatly appreciated!