PHP mail - sendmail - and SMTP mail, unable to reach users in domain

Switched this domain to Cloudflare 2 years ago and it was working fine until recently. Hosted on Hostgator; their support insists it is DNS related. Mail server is NOT at Hostgator, it is offsite, and it is not proxied (it is gray cloud).

using sendmail via PHP mail(), with or without headers, I receive mail sent to other domains, i.e. [email protected], [email protected], etc. However, sending to [email protected] is never received.

Attempted to use PEAR::Mail and another SMTP mail option, and when attempting to reach mail.THISDOMAIN on any port, the response times out.

Attempted telnet and ssl to mail server and similarly timed out.

Has anyone had this issue? Have a solution or suggestion?

Thank you for reading.