Basically this is all about ARC!
And the lack then of a DMARC policy - especialy as GMail and Yahoo are basically now enforcing this.
Look on DMARC as the policeman of the email world - find out your score here.
EasyDMARC | DMARC Journey Made Simple
and follow the instructions from there.
If stuck (as this can become complicated quickly) engage our support in this instance to sort this out for you and get your emails in your Inbox.