How do you send these emails? from the Webmail or from a Email Client?
Please send an Email to GMail. Then check the Email header.
SPF:
DKIM:
DMARC:
Do they all 'PASS'?
Also check the,
Received: from xxx.xxxx.tld (xxx.xxxx.tld. [IP])
Does this IP's rDNS match the sending host (xxx.xxxx.tld) ?