One of the security features that Gmail offers is to indicate if it couldn't confirm that a message is coming from the person who appears to be sending it, by showing 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 [domain] actually sent this message (and not a spammer)"
This is useful to prevent phishing attacks, although sometimes legitimate senders may still show as unauthenticated. The cause is that the sender's domain is missing an SPF record.
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” on the search bar:
You can also check it at Emails → domain name → DNS Settings:
When the SPF record is correct and propagated, you will see it as Active:
If your domain is pointing elsewhere by nameservers, the SPF record needs to be set up from the service to which you are pointing your domain. You can find more information on the SPF records for Hostinger Email or cPanel email here:
If you're using Titan Email, you can check this article: How to set up a domain for Titan Email?
Any DNS record changes trigger propagation which can take up to 24 hours to fully propagate worldwide. Your messages may still appear unauthenticated until propagation is completed