Gmail shows a red question mark as a profile photo for non-authenticated email senders:
When you hover over the question mark icon, this message is shown: "Gmail couldn't verify that [mydomain] actually sent this message (and not a spammer)"
The cause is that there is no SPF record on your domain's DNS Zone.
For domains that are pointed to Hostinger by nameservers, the SPF record is added by default. You can find it in your DNS Zone Editor by inserting “spf” to search:
You can also check it at Emails → domain name → Email Delivery:
When the SPF record is correct and propagated, you will see it as Active: