cURL error 6: Could not resolve host: email..amazonaws.com - Mailster

I use the WordPress plugin of Mailster to send emails out to the subscribers through Amazon SES.
I've been doing this for 3 years with no problem at all.
I've recently moved my hosting company to Hostrator since 4/20/2021 and after a few contacts with Hostgator tech support, this started to work fine until three days ago.
All of a sudden whenever I try to send an email I get the below error message (underlined) and of course, I cannot send out email.
Couldn't send message. Check your settings!
cURL error 6: Could not resolve host: email..amazonaws.com
Hostgator tech support tells me that all Amazon SES Domain Verification TXT Records and Amazon SES DKIM Record Set have been reflected as per my request and they don't see any odd settings on their side.
Any thoughts?
TIA.
All the best.