Hello @7362-Monster
Well I had the problem where jre that comes with katalon studio didn’t have certificate installed. So here are the steps that fixed my problem. Please not that I will not post how to do every step since it will take too long. You can google how to do every step, it should not be too complicated.
1. Disabled my antivirus
2. Allowed inbound and outbound mail ports on windows firewall (ports 465 and 25) *
3. Fetched the ssl certificate of mail server, using openssl
4. Imported certificate into keystore using keytool **
5. I Allowed less secure apps on my google account ***
6. Your email should work now
7. You should turn on your antivirus back on when you finish, or decide to create an exception for Katalon studio
IMPORTANT NOTES:
* - 465 and 25 are common used ports for email but it does’t have to be true in every case. Please check ports on your mail server.
** - You have to import them into the keystore of the JRE that Katalon studio is using. So you should import it into KatalonInstalationFolder\jre\lib\security\cacerts
*** - Only required if using gmail account
I hope that this can help you in solving your problem
Best regards 
Ismar