I suggest checking the domains which are having difficulty against the new EFF Transport Layer Security website:
https://starttls-everywhere.org
If those domains don’t allow properly secured incoming connections, some outgoing SMTP servers might not allow a connection to send mail, since the transfer could be compromised.
Outgoing email servers may block connections to IP addresses or servers which are thought to be insecure. This is to prevent malware or bad links in email messages connecting to spammer servers. So be sure that the IP ranges are not on any block lists.
Bill