Why does my iPhone email always say its sending emails when I check it?

Everytime I open my email it says
'Sending 1 of 17… '
What exactly is going on there

Check your out or send box. Maybe you have a huge file that can't be sent.

It's either what the first answerer said or maybe your network doesn't work well all the time so it's always trying to send the emails you've tried sending before but didn't work or you thought didn't work? Can't think of anything else