My sent emails are not showing up in the sent folder any suggestions?

I have an iMac, 13,1 model, using OS X Yosemite. It has 8 GB of memory, a 2.9 GHz Intel Core i5 Processor.

The other day I could not send emails. I looked up why this might be and can now do this on my iMac (but not my iPhone or iPad). This issue with the iMac now is that I cannot see the emails I have sent. They no longer show up in the "sent" folder. I have tried a number of things. Some of the things I tried included following a thread from someone to go into the Mailbox and sort things out there. However, much of what was suggested was greyed out. I also tried going into settings, preferences, general as it was suggested to someone else that they check the sent messages, couldn't find this box to check but all others were checked anyway.

Any suggestions?