Sorry for asking this but I was trying to contact support, basically to request access to custom CSS, and uh Gmail says that the recipient doesn't exist (from the mail delivery subsystem it says).

Just for informational purposes (I know this is not gmail's support forum), the reply from Gmail thing says the following:

Your message wasn't delivered to support@itch.io because the address couldn't be found, or is unable to receive mail.
The response from the remote server was:
552 5.1.1 Mailbox delivery failure policy error

I am a bit confused at what's going on.

The same thing is happening to me. I can’t send an email to support.

Same happened too. They literally didn't answered me for half month

Can confirm the same happened to me from two separate email accounts. I was trying to contact them to get a refund from an accidental double purchase I made on something half a year ago but it's a bit hard when the email server isn't even receiving the email. It seems like it's misconfigured to not receive emails but it seems it's still sending them because I'm getting receipts just fine.

Yeah, I tried this morning from a different email host, as in not with Gmail.

Your email could not be delivered to one or more recipients. See below for the delivery error and email.

The most common reasons for undelivered email are wrong recipient address or wrong configuration on the recipient mail server.

support@itch.io: host mx.zoho.com said: support@itch.io Mailbox delivery failure policy error (in reply to RCPT TO command)

I removed numbers from the error message, because those looked to me like IP addresses.

So it seems. See this related topic.