There are three methods attempted for processing the emails the plugin sends out. The first is the plugin’s send_mail() function. The plugin will try to send out the email using this function. Some servers will not permit the plugin to send out the mail using the send_mail() function. On these servers, the plugin will then attempt to use the standard php mail() function. Here again, some servers have restrictions in place that will prevent even the standard mail() function from processing the emails.
If send_mail() fails and mail() fails, the system will attempt to use SMTP to process the emails. The SMTP feature is experimental at this point in time. It is not known yet if it has worked for anyone as no one who might be using it has reported back. If you are using the SMTP option successfully or trying unsuccessfully to use it feedback is invited.
If none of the three methods is working for you there is something, likely on your server end, that is preventing the emails from going out.
One known issue that has resolved this problem for a few people:
Certain hosting providers do not relay emails using off-domain email addresses
Some servers appear to require the sender email address to match the outgoing domain. In other words, if your domain name is awpcp.com and your wordpress admin email is firstname.lastname@example.org, your server (if you are on a server with this particular issue) will not send the email. Changing to an email like email@example.com fixes the problem. (Of course awpcp.com is only an example. It would be replaced by whatever your domain name happens to be)
Your contact path value as set in SEO Settings could be wrong
If your problem only relates to emails when someone is replying to an ad, you are probably dealing with a path issue. The contact form is probably not getting a correct ad ID because the value set in Classifieds->Manage General Options->SEO Settings under the heading “If contact page link not working in seo mode change value until correct path is found. Start at 1”, is not correct.
You can verify if this is the problem by viewing the source code when on the contact page to reply to an ad. Look for the hidden value with name adid.
<input name="adid" type="hidden" value="4-Dolce--Gabanna-dresses-for-sale" />
In the example above the value is set to “4-Dolce–Gabanna-dresses-for-sale”. This is not correct and has occured because the setting in SEO Settings for the reply to ad (contact) form was not correct as explained in Reply To Ad
In the above, the system cannot retrieve the ad information because it does not have the correct ad ID, so while it appears to process the contact form, it does not.
If you are not receiving the contact emails but receiving all other emails, please check to see if the problem is as described above by viewing the source code to see what value appears in the value slot for the hidden field adid.