Webform Mail not arriving at gmail address I'm having problems with the mailing of the Webform Module. 

I have an online website where you can fill in a form. The results will be sent to the email-address the user gave in and the user of the site. 

When the user fills in an email like myname@hotmail.com or myname@mycompany.com there are no problems. They receive the results perfectly. But when they use a gmail address like myname@gmail.com they DON'T receive the email. So it never works with a Gmail address.

The user of the site (who normally also receives the results) then gets an error like this:


  Subject: Undelivered Mail Returned to Sender


This is the mail system at host myhost.com.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                 The mail system



  : host gmail-smtp-in.l.google.com[ip-adres]
    said: 550-5.7.1 [ip-adres      12] Our system has detected that this
    message is 550-5.7.1 likely unsolicited mail. To reduce the amount of spam
    sent to Gmail, 550-5.7.1 this message has been blocked. Please visit
    550-5.7.1 http://support.google.com/mail/bin/answer.py?hl=en&amp;answer=188131
    for 550 5.7.1 more information. 44si1984154eef.10 - gsmtp (in reply to end
    of DATA command)


What could be the problem with this?
From the link in your bounceback message:

"A message might be blocked if it contains suspicious-looking or spammy text or if the sending IP has had a history of sending unsolicited messages."

Either one of these possibilities might be the issue.  To rule out "suspicious-looking or spammy text" you should look at the emails your webform generates.  Is there any text they have in common that might be triggering gmail's spam rules?  If you send such a message to a gmail address manually, do you get the same bounceback?

The more likely explanation is that the sending IP has had a history of sending unsolicited messages.  This is especially likely if you're running in a shared web hosting environment.  If this is the problem, there's likely little you can do about it, other than asking your host to move your account to a different server.