That is correct. When the email client tries to access web base email accounts and you’re connect to Secureling the IP address won’t be recognized and will deny connection as you experienced.
If you then go online and directly sign in while connected to Secureline then it also can resolve itself.
It’s a security layer by web based email accounts to deny unrecognized sign in perimeters. 
The receiving of a notice and “send code” options will also ocurr.