Ok i`ve seen this 100 times around the whole forum and it is time we put an end to it.
The forum is experiencing problems with the smtp ( sendmail server ) That means that every action which required or was followed by a email being sent to a user from the user WILL be performed, but the email will not arrive to the user.
That means, you can still send pm, you can still reply to topics,you can still register BUT you will not receive an email from the forum if you receive a pm, if you ticked to be notified in case of a reply, when you register to the forum etc.
The only problems regarded to this problem are the annoying error message you get which can easily be ignore, AND if email activation is required for registering. In case you do actually see this post, and cannot register on the forum email firstname.lastname@example.org
and tell them you can`t register on the forum and why by quoteing this post.
This having been said, i hope i won`t see posts or topics opened regarding that error message.