Forwarded emails rejected by Gmail

Hello,
I have a problem with messages that are forwarded from module Ajax Contact and this from toukoul.be domain.
It seems that messages are not authenticated when they are forwarded from the contact form, strange because the form has been running smoothly for several years...

To be more precise, form's messages are forwarded from info@mydomain.be email address to myemail@gmail.com email address, but I don’t know why now all messages are blocked by Gmail (only these coming from the contact form) and I have this type of message:
www.dropbox.com/s/mxv9yhrrxovcnc4/emailrejected.zip?dl=0

I contacted my web host (SiteGround) who certified me that the domain name has all the appropriate email authentication records (SPF, DKIM and DMARC).
He suggested me to configure the contact form to authenticate the forwarded emails.

I really don’t know where to start… how to prevent that messages forwarded from module Ajax Contact are rejected by Gmail?

Thank you for your help,

Lorenzo

.... finally I tried another solution: I deleted email transfer from my host and I tested sending the message directly from Ajax Contact module by inserting Gmail address in “Blind Copy” field .... the message is not delivered...

Thank you for your help,

Lorenzo

Dear Lorenzo,

Authentication problems are usually solved by setting the Mailer to SMTP in Joomla Global Configuration. You must also enable Authentication. I also recommend using Encryption as well.

For more information, please read item #5 of this article .

Let us know if the problem persists,

Hello and thank you for your reply,
I just found an alternative to work around the problem, instead of forwarding emails via my host, I added this email address in my Gmail accounts in order to check emails from another account and it seems to work well.
I come back to you if I decide to adopt your solution.

Thank you again,

Lorenzo