E-mails not being sent. Error: 554 5.7.1 Recipient addres rejected

[Plantje]

In the log of the forum I see a lot of these error messages:
Ran into problems sending Mail. Error: 554 5.7.1 <someuser@someisp.nl>: Recipient address rejected: Authentication required

Obviously I removed the actual E-mail address. Strange thing is: this is happening with all sorts of recipients. GMail, Ziggo, Hotmail, Telenet, Caiway...even Belgian recipients.
Next to that it is happening with notifications of topic replies, personal messages, notifications of new topics etc.

If I resend the mail queue I get one of these error messages for every single mail.

Is this something on our end? Or on the recipients end? What can I do to trouble shoot this further?

[Arantor]

Means the user that is supposed to get an email has an invalid email address. All you can do is go to the user's profile and untidy notifications for them.

[Plantje]

Could it be one user in the whole mail queue?

I mean: my own E-mail address was in there as well (I deleted some items from the queue). And it is the same E-mail address that is used on this forum... (And here I did get a notification E-mail).

In the meantime I have sent the users for whom I still had mails in the queue a direct E-mail from my personal account...nothing undeliverable back yet...

[Arantor]

The 554 message is it telling you the message was undeliverable. Whatever address it was trying to send to does not exist. That's all there is to it.

[Plantje]

This is really too weird then... I am sure at least one of the addresses (mine) is correct.
What does the part "authentication required" mean?

[Plantje]

None of the E-mails that I sent from my account to the effected E-mail addresses has bounced. I'm still wondering where authentication is required. Is that an authentication on my end or on the recipient's end?

I did notice that the smtp port is set to 25 where this should be 587 according to the website host. However, I still don't know the SMTP's credentials. Can I change the port number without having to enter the password again?

[Plantje]

Turns out the web host had made some changes to their SMTP.

Strange though, that this is the error message received...

Anyway: fixed for now!