Very slow to send emails; also slow to synchronize and receive emails

I’ve been using EMClient for a few months and it has been working just fine. But now I’m having problems…

Yesterday, I started having problems sending emails: they either took a long time (maybe 10 minutes, maybe more) to send, or they didn’t go at all. Now today, I’ve noticed I’m having problem synchronizing and receiving emails.

I’m also getting an error message:
[Exchange Web Services] An attempt to connect to … failed. This could be caused by temporary server unavailability or incorrect settings. Do you want to check the settings?

The settings seem to be correct. And I assume they are as eventually (most) messages are received or sent.  

I see others have had very similar problems but I couldn’t see any answers that help me.

This unreliability is a problem for me as I use this for work - if anyone knows what the problem might be, please do let me know. Otherwise I’ll need to switch to something more reliable. 
 
Thanks in advance!

I am sure it is frustrating.

I suggest you create a support ticket at https://support.emclient.com, and include your errors. They are usually quite quick to respond.