[ic] Strange "Unable to send mail" error

[interchange-users@icdevgroup.org (David Bordas)]

Hi all, i'm back and i still have got a problem with a production IC server
:(

In fact, akopia works great but sometimes ( about 1% of error ), order email
aren't send but order can be seen in the admin area ...
In the /catroot/error.log i can see something the order email print there
and as error :
Unable to send mail

I don't see anything in the /var/log/maillog and /var/log/message.

Perhaps it's that server was loaded but i'd like to have some more infos
about this.
I saw in the list that someone that had got the same problem before, but i
can't see any help reading theses posts.

IC 4.8.2

Thanks again
David