Mandrill extension for Osclass enables to use Mandrill mailing services (by MailChimp) to be used to deliver you mails.
In case you have problems with delivering your mails, there is big delay while mails are delivered or your mails are coming to spam folder directly, it is better to use external mailing service provider.
Mandrill plugin is easy to use and require to setup only SMTP username and SMPT password, everything else is then covered by plugin itself.
There is free quota of 12 000 emails being sent per month for free.