You did not mention what email service you are using ?
A lot of services i.e. Gmail, Outlook, etc have increased or are increasing security, there has also been changes made to later version of nopCommerce to work with these new connection requirements
cosmeticishop wrote:I contacted our hosting that also manages the mail, which has carried out checks on both the mail configuration and the SSL certificate, which are both correct and working, and they replied this:
What do they mean when they say it is working ?
You have tried to obvious things of restart the server / application