Cannot send mail from gmx

[Dal123]

My apple mail was fine till a week ago when I had to restore everything. I am trying to configure my apple mail to work through gmx mail. I'm receiving emails fine however cannot send. I get this error:

"This message could not be delivered and will remain in your Outbox until it can be delivered.
The sender address username@gmx.com was rejected by the server"

Just need to know how to fix this as it's quite important for my business, any help will be well appreciated.

 

[the Western zoo]

It might be because your ISP has a SMTP/port 25 blocker, forcing you to use their SMTP-server.

I use GMX with Mail on my MacBook, but since my ISP is reserving port 25 for their SMTP-server I use SSL over SMTP because GMX's SSL over SMTP rund on port 465. (the server is mail.gmx.net)

Edit: the reason I chose to use the GMX SMTP-server is that it will work no matter which WiFi I connect to...

 

[Dal123]

Sorted

Thanks for input; sorted now. All I did was SSL thing and entered my password and username through it. All working perfectly now.
Thanks

 

[Dal123]

Somethings up

It's been working fine for a week. Now all of a sudden it's happening again. Nothing has been changed. Why is this happening? This is a big problem, I'm starting a business and need to send these emails out to try to get my own work.
Has anyone else got problems like this?