So, I mistakenly deleted my outgoing mail server on my Odoo 10, and I immediately created a new one. and after clicking on TEST CONNECTION it show the validation message below.

Connection Test Failed! Here is what we got instead:
-2
Name or service not known

Also I got this error message below anytime I click on Save Button or Approve Button on all my installed module.

"The operation cannot be completed, probably due to the following:
- deletion: you may be trying to delete a record while other records still reference it
- creation/update: a mandatory field is not correctly set

[object with reference: ir_mail_server - ir.mail.server]"