@Iulian_Olaru,
It is also convenient to check your server´s reputation on senderbase.org.
This will give you an indication whether your server has a bad reputation, that Gmail acts upon.
It can be added that an entry in a DNSBL lists, such as those of spamhaus.org, can also be checked, but quite often Gmail does not really act upon them, since Gmail primarily acts upon specific content in the mails and/or specific DNS settings of the mail server (note: this implicitly explains why the senderbase check is relevant).
Anyway, in order to prevent mails from the website contact form to end up in the Gmail spam folder, just do the following:
1 - create a (separate) mailbox on the server hosting the website (preferably a domain, different from the website domain),
2 - on the mailbox from step 1, create an automatic forward (in Plesk) to your Gmail account,
3 - set the contact form to mail the reactions from the website to the mailbox from step 1 (i.e. a local mail delivery, not giving any spam related errors)
and this way, one achieves some advantages:
- in the best case, Gmail will mark mails as spam, based upon the content (note: if so, it should suffice to "tell" Gmail that these mails are not "spam"),
- Gmail will NOT mark mails as spam, due to some "mail relay" related cause (note: this is very likely the culprit for the spam marking in your current situation),
- a simple backup of mails is maintained in the local mailbox (see step 1), being a replica of mails sent to Gmail,
- mails can be checked twice for spam/virus on the server hosting the website (incoming from the website, outgoing when forwarding).
In short, the above method allows to reduce the impact of spam on the (local) server and/or reduce the spam markings in Gmail.
As a final note, you should VERIFY that the website contact form is using the mail server installed with Plesk (qmail/postfix) and NOT sendmail (or similar), given the fact that, with sendmail (and similar), you are very vulnerable to "mail relay" related spamming by other parties than yourself (i.e. hackers).
In my personal experience, I very often have to observe that the Gmail related spam markings and/or bad reputation on senderbase and/or DNSBL listings are often the result of other mails (by spammers/hackers) and not the mails of the Plesk customer.
Be aware of the fact that your server has to be secure, since "bad reputation" in general can cause all mails from your mail server to be blocked and/or marked as spam.
Kind regards....