Mail server permanently rejected message

[artroom]

I am sending out some newsletters, but am getting bounce backs with the following error:

Your message was rejected by <isp.name.here> for the following reason:
mail server permanently rejected message (#5.3.0)

I can send via my normal email application to these same people without problem, but anything i send to them via MBM bounces back with this error.

The message also says "This type of error is usually due to a mis-configured account or mail delivery system on the destination computer; however, it could be caused by your message since some mail systems refuse messages with invalid header information, or if they are too large."
Does anyone know what this might be? I've tried adding the same X-Mailer header as my email app, but this didn't help. File size shouldn't be the issue, so i don't know what to try..
I really need to sort it out as the people affected are my main clients...

thanks

[stanbusk]

What MaxBulk Mailer version are you using?

[artroom]

I'm currently using 5.1, but I think i had the issue on an older version as well (5.0x)

[stanbusk]

Have you tried to remove all custom headers?

[artroom]

I believe so, yes.

My ISP said it was because I was sending via them (using Authentication), but using a different from and reply to address.
I don't believe this is the case though, as that is exactly how my Entourage is setup and I can send emails to the same recipients fine from there.

[stanbusk]

MaxBulk Mailer and Entourage send the messages exactly the same way so I guess it has something to do with the message contents or the number of messages you are sending at the same time. Have you tried to use the group and interval functions? They are handy to simulate human deliveries.