ERROR CODE :554 (certain recipients are bouncing my emails)

ERROR CODE :554 (certain recipients are bouncing my emails)

I had set up my SPF, DKIM, and DMARC properly as far as I know, and they look proper according to mxtools and some other online checking tools.

My mail does go through to gmails (although still was in spam at first), but certain domains bounced me.

I'd really appreciate some assistance,

best,
LF