Laravel: All RCPT commands were rejected with this error: 503-relay not permitted, authentication required

I've made a website with Laravel which uses email to notify users of certain things. For testing on my personal computer, I used my gmail account for sending email from te website, which worked perfectly fine. Now I want to launch the website for the public and for the final version I want it to use noreply@mydomain to send email instead of my gmail adress.

Therefore I've set set the environment variables in the .env file like so:

MAIL_DRIVER=smtp
MAIL_HOST=mail1.webyte.eu
MAIL_PORT=587
MAIL_USERNAME=noreply@mydomain
MAIL_PASSWORD=mypassword
MAIL_ENCRYPTION=tls

The guy from my hosting service told me that this should be correct, however, when I try this, I get the following error:

Swift_TransportException (503) Expected response code 354 but got code "503", with message "503-All RCPT commands were rejected with this error: 503-relay not permitted, authentication required 503 Valid RCPT command must precede DATA"

I've been searching the internet for a few days now and couldn't find anything that works for me...

Does anyone know what I should do here? Thanks!

via Chebli Mohamed