(SOLVED) SMTP Mailer can't send mails but my mail client with the same credentials can

Hi there

We use MailJet as SMTP server. It has worked well before but suddenly it doesn't When sending test mails we get the failed message.

But when we send an email from a mail clients with exactly the same credentials it works fine.

Any ideas what can be wrong? We are on B1

UPDATE: we Can send if we use the mailserver on our hosting, but not if we use an Extermal SMTP provider

SOLUTION: Somehow it works when you select port 443. No clue why. I just found the suggestion in a blog post after giving up getting useful help from our hosting provider. LeonidS , Thank you so much for helping narrowing in the issue

All the best

Finn