have a look in account settings > Copies and folder. Is the spam folder set to receive a copy of sent mail?

Or are you asking about when you email yourself? Many providers consider that to be spam and filter it accordingly. But Thunderbird has no such option.

thank you Matt, i'm thinking it's happening when i email myself from my phone

Try creating another email account. GMX is quick to set up, and email yourself from that using your phone. I think you will find the emails no longer go to the spam folder. I have an account with them and I am happy with what they offer. No intrusive adds, no privacy intrusions. Just a plain mail service with a calendar that just works.

thank you Matt, just signed up, looking forward to ditching Google's gmail