Trouble sending mail from Dreamhost to REG123

Evening, I have my domain with REG123 but have my site hosted with Dreamhost, initially I set up my DNS records on REG123 to reflect this and then placed my MX records for REG123 in Dreamhost DNS Custom MX settings. All was well for 24 hours, with my mail being sent from Dreamhost to REG123 and then forwarded to my other mailboxes. However after 24 hours it all stops working and nothing is being sent to my reg123 mailboxes, can anyone help? I;ve tried the so-called 24/7 support at reg123 which appears to be non-existent, even though they say they have a ‘chat window’, I;ve now had to open a ticket but still haven’t heard anything back 6 hours after my query. Any help or advice is appreciated.