ERROR CODE 550, 521, and 554

I am getting mail bounced with these error codes when I send from my iphone or ipad.

They don't bounce when I send it from web mail.

This started recently when I switched to using m.sync.zoho instead of IMAP and SMTP.

And I have tried setting up SPF details in a TXT record for my domain, without success.

I raised a support ticket on this several days ago but received no answer.

Can someone help?

More Detail:

I have Zoho Mail set up with an address at my personal domain.

Outwardly I use a yahoo email address, so all incoming mail is forwarded from Yahoo to my Zoho domain mail,
and all outgoing mail is set to Reply To the yahoo address.

As I said, this all worked fine when I used IMAP and SMTP for the iphone and ipad, except that I wasn't getting Push notifications of incoming email. So I tried switching the iphone and ipad to use ActiveSync instead, following the instructions here: https://www.zoho.com/general/blog/announcing-zoho-sync-for-mobile-devices.html

Incoming mail is working fine. But when I try to send mails from iphone or ipad, I get bounces from some email addresses. The error code seems to vary depending destination.

GMAIL: Error, ERROR_CODE :550, ERROR_CODE :5.7.1 Unauthenticated email from yahoo.com is not accepted due to domain's

AOL: Error, ERROR_CODE :521, ERROR_CODE :5.2.1 : (DMARC) This message failed DMARC Evaluation and is being refused due to provided DMARC Policy

YAHOO: Error, ERROR_CODE :554, ERROR_CODE :5.7.9 Message not accepted for policy reasons

Not all email addresses bounce.

And nothing bounces if I send from my Zoho web mail.

Searching the forums I found this suggestion:

https://help.zoho.com/portal/en/community/topic/error-code-550-error-code-5-5-2

Following these instructions I have added a TXT record to my DNS settings listing both zoho.com and my personal domain as permitted domains. That was 24 hours ago, so it's had plenty of time to propogate. But I am still getting the mail bounces.

Thanks in advance for any help.