Returned mail too many hops???

Since there is a great deal of knowledge in this group I want to ask you 
a question.

I'm with provider A, sending mail (an e-zine) to someone who has an 
account/domainname with provider C I get the mail returned from provider 
B with the message ' Returned mail: Too many hops 26 (25 max)'

Does anyone know what that means?

It happens now for the past months (since I started the magazine) and it 
is the first time it happened ever. 

I allerted the postmasters/ webmasters of the sites B and C of this 
problem but they do not react or say stupid things like 'It is because 
you post mail in HTML format' (which I do not but what is the 
difference?) or they assume I'm with provider C (but my adress states I'm 
with provider A) or they do not react at all.

Because Im a bit tired of this arogance (thats how I explain it) I would 
like to know what it is so I can tell these 'professionals' what to do...

Or maybe it is not the fault of providers B and C, it is the fault of my 
own provider, but they are not the ones who sent it back.

Jeroen Goulooze