When email is viewed through the Gmail website the sender's avatar will show next to the email details, if the email is detected as not being authenticated for any reason then a question mark will show there. The likely reason for this is that your domain doesn't have an SPF record.
An SPF record can be set, which specifies which servers are valid senders for your domain (as any mail server can specify any e-mail address as a "From" address). Have a look at the following
article for more information on the record to setup.
This should then prevent that error from appearing (keep in mind that DNS records can take up to 24 hours to propagate).
