How can I fix "Gmail couldn't verify that [mydomain] actually sent this message (and not a spammer)"?

Gmail shows a red question mark as profile photo for non-authenticated email senders

When a Gmail user moves his mouse over the red question mark icon, the following message is displayed:
"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 Editor.

You can follow the instruction on this link to configure an SPF record.