Hi,
First, note that there are two email servers that play a role there:
1. The email server of your web server which sends the email for your website
2. The email server of your recipient domain name which receives the email
Both can be the same email server if you're sending an email from your website to an email address of the domain name of your website.
So this sentence is a bit vague:
They have checked the logs and they say that there is no such email received to the mailserver
Are they talking about the email server sending the email ? Or the email server receiving the email ?
But as I understand you tried with another email address and it worked, and even for the email address where it didn't work, you still got the email in the email history of HikaShop. So the problem is not with HikaShop. You'll have to see with the settings of the sending or receiving email server and we won't be able to help you further with that matter.
Regarding the log they gave, it says that the receiving email server has a SSL certificate that has been self-signed and not signed by a trusted authority. So I would wagger that you're using an email address of your domain name which isn't using a properly configured SSL certificate and thus the email sending is refused.
You'll have to check that with the person whon put in place the SSL certificate.
There is a free service now available to sign certificates so it doesn't cost anything (beside the time spent doing it) to properly sign a certificate:
letsencrypt.org/