Asterisk sip.conf provider registration problem

Hello all,

It is my first post, so please be tolerant.
I have problem with register my SIP provider in Asterisk

My provider requires in authentication field (authuser): ‘user@domain’ , so when I put in register line:
register => user:secret:user@domain@host/extension

I received following message:
[Jan 22 08:37:00] WARNING[13228]: chan_sip.c:6237 sip_register: A reserved character (’@’) was used in a “register” line. This registration will not occur

I did not have any problem with registration in previous Asterisk version (1.4) for the same SIP provider. I putted exactly the same and registration was properly.

Is there any solution for my situation?
Could you be so kind and provide me some example of configuration for similar issue ?

Thanks in advance