iMac can't send emails? Need Help

On my "iMac" I can receive email but can't send from it. I'm using the email that came with the computer.

It was fine last Wednesday, then started refusing to send the emails. Then it was fine on Thursday afternoon, but stopped sending again on Thursday evening.

The error coming up says the server cannot connect.

I contacted the server company and they tell me it's a MAC/Apple problem. I contacted MAC/Apple and they told me it's the server problem.

Strange thing is, that everything works fine when I use the PC for emails.

Both are hardwired from a "D-Link".

Suggestions?

 

I checked settings and all is the same. Nothing has changed. My server and Apple both took over my computer and said their side works and blames each other.

I'm really getting pissed. I might have to go back to my old PC.

 

Here is something strange.

I just tried the email again and it allowed me to send five emails.

However, it blocked the sixth email and now it doesn't send again.

What is going on?

 

I did answer Miles.

My mail is the one that came with the "iMac". It's been working fine for two years.

My provider is the local phone company. They and I have gone over all the settings. They say all is working fine.

I have gone over the settings with MAC/Apple tech people they say all is fine.

I think the problem is with the MAC. It's probably not "in the manual/rule book". We have to look at something outside of the box.

I was talking with a lady in Southern Ontario that has been having the same problem for months.