Am I being paranoid or do I have a right to be worried?!

I'm engaged and getting married at the end of the year. I was sending something via email earlier and was having trouble so asked my fiance if I could use his email account instead. He was very hesitant but gave in. I sent the file I had to send and whilst waiting for confirmation from the sender that the email has sent I saw emails in the sent folder from Craigslist. I opened one and it was saying something along the lines of 'Hi, I am very interested in you. I have a partner so is difficult to find the time but should be able to do Friday. Looking for an experienced woman.' it says it was sent via Mobile and was sent at the end of 2017. When I confronted him about it, saying id not opened it and had just seen the beginning, he went mad, logged me out and asked why I was reading his emails. I said I wasn't and that I was waiting to see if the email had sent and I just saw the bit of it there and I said it's just a bit weird and he asked if I was accusing him of cheating. I said no but then got angry myself and he ended up saying he bought something from Craigslist a year or so ago and since then he's been getting virus type emails and calls from Craigslist. I don't know anything about the site myself but I just don't know whether to believe him. I'd believe him if it was in his junk folder or his inbox but it's just weird that it's in his Sent folder and says sent via mobile. Please help 😭 I can't stop thinking about it and feel like I'm over thinking it all!!

He spent years trying to win me over and we're such a happy and lovey couple now that we're actually together! We are engaged and have a baby together and I'm hoping this is all just a misunderstanding, more for my baby's sake :(