Emails won't send and eM Client runs slow

I’ve been trialing eM Client for about 5 weeks now. Mostly it has run fine. I use the latest version (6.0XX) - the system says there are no new updates. I use eM Client to connect to my Exchange account at work. I have verified their server is working fine for everyone else at work. Lately eM Client has started to run slower and slower when sending and receiving emails. And just last night I can’t send any emails - though I am still receiving them. When I try to send, I just get the spinning wheel for about 5 minutes - then nothing - it doesn’t go to the sent or outbox tray - it just disappears, and the recipient says they never got it. Obviously this is critical in my decision to purchase…