Why aren't my emails being sent?

Good evening,

I've been using the Mail app on my iPhone for a while now. Today, however, I noticed that the replies I send to emails aren't in the Sent folder. I then looked on my PC and discovered under one of the emails: "Reply deleted, click to restore" (or something similar). When I restored it, it was sent. But why aren't they sent normally? Also, there was always a gray arrow next to the contacts I wanted to send the emails to, so I assumed the reply had been sent.

Thank you in advance