email is confusing.
your post is confusing.... or i'm just slow...
what what i gather, some email goes through, other emails associated with yahoo/at&t/sbcglobal do not? is that correct??
with regards to this "masked" email hypothesis, i would refer you to your own 1st paragraph. in other words, i think very little about this hypothesis.
you have a website; it has a domain; you have a host that provides email services; but now your domain now utilizes gmail. do i have that correct?
you are currently utilizing your hosting company to send mail from your website. some of that mail fails. if i gather that correctly, then your hosting companies' mail server has a bad reputation, and certain domains are blocking your hosting companies mail servers from sending email to their customers (confused yet?). this is not unusual.... i am currently dealing with a similar situation.
on the cpanel error, its pretty clear that apple icloud is blocking you from sending email due to a bad reputation. do you Make use of Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM) to authenticate your emails? its a good idea.... refer to their link:
https://support.apple.com/en-us/HT204137
i would suggest utilizing the gmail servers to send email as opposed to your hosting company. does gmail allow you to do that? i would think that they do...
either that or get SPF, DKIM and DMARC established for your domain.
if i have misunderstood anything with regards to your post, i apologize in advance. just trying to help, and as i said email is confusing!
good luck.
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