Here's the thing. In the last version we also updated PHPMailer (due to a vulnerability). This version of PHPMailer verify the validity of the certificate (it's the correct domain, it's not expired,...) of your email server by default. This is how things have to be, if you use a certificate but it's expired or issue to the wrong domain, it shouldn't work. You could disable this check (in the code) but we do not recommend it.
So, what is happening? Well, some hosting are using the wrong certificate when their clients connect to their mail servers. Your hosting company probably has something like "mailserver3.hostingcompany.tld" and you could probably connect to the same server with your domain in a nicer way with "mail.yourdomain.tld". The problem is that your hosting company (probably) only issued a certificate for mailserver3.hostingcompany.tld, so when you are connecting through mail.yourdomain.tld, the certificate belongs to a different domain (but same server). and thus failing to connect and send any email.
There several ways to fix this, the first is to ask your hosting company to do things properly and use the correct certificate (they are free!), you could use directly "mailserver3.hostingcompany.tld" as your email server, so the certificate is correct, or you could modify Osclass to disable the verification (which we do not recommend). The modifications are explained here :
https://forums.osclass.org/3-7-x/update-to-3-7-3-my-findings/msg149561/#msg149561The problem might be different, but this seems to be a probable cause