E-mail received but loader is stuck and no confirmation message

I have the latest version 5.0.1 installed and I receive e-mails submitted through the contact form, however the loader is stuck after clicking on the “send” button. This gives the visitor the form the false impression that his e-mail was not submitted as the confirmation message never appears. Neither does there appear any error message saying that the e-mail could not be submitted.

My website is hosted on domainX.com, however I use Google Apps mail server (i.e. domainY.com) to send e-mail as configured by the latest version of the WP Mail SMTP (1.2.5) plugin. Hence, the sender’s email address domain is not the same as the website’s domain.

When downgrading the contact form to version 4.3.1 (before the introduction of the validator), I experience the same problem as described above.

I have another website hosted on domainY.com, and when I use the contact form with e-mail sender being on the same domein as the website (i.e. domainY.com) I do get the confirmation message.

How can I solve this bug that there is no confirmation message (either successful or unsuccessful)?

Thank you.