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Try doing a search for 'unable to send the activation mail ' - you will get quite a number of results from the forums - it could be a number of problems and i'd rather not guess. If you can't resolve it yourself you need to also follow the indtructions in the 'read before you post below' as we will need more specific info about your site and software versions. Good luck!
PS:I always say this
New users should always checkout the following:
1. Documentation - linked on the left. Specifically the administration manual
2. The FAQ section - linked on the left - specifically the sections on gloassry, beginners first steps, modules, blocks and themes. There are links to visual tutorials here.
3. The search function on the top right.
Please check also the read B4U post link below if you have problems.
For Visual tutorials please go
here