Failure to reply to a Craigslist ad is usually the result of Windows being set to recognize the correct email program as default. Other problems may include a listing not configured with the means to email inquiries, or that the ad in question has been removed.
Default Mail Program Error
Errors with the default mail client are some of the most common causes for reply errors on Craigslist. Usually, this is because while you may use one email program, Windows is set to look to another one as the default. When the default program isn't configured with your email, you can't send messages as replies. There are a few ways to deal with this problem: Resetting the default email client, using an alternate option or copying the email address directly into your preferred client.
Set a New Default
Press Windows-X and select Control Panel.
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Click Set your default programs.
Select your preferred email program and click Set this program as default.
Use an Alternate Option
The Reply button also has several alternative options accepted by the ad's poster. Gmail, Yahoo Mail and Outlook.com are all possible alternatives that the poster can suggest. Clicking any of these options launches the mail services in question, with the listing's subject and the Craigslist email address already entered in the appropriate fields.
Copy and Paste the Address
The Reply button also offers a link to Craigslist email address associated with the post, which can be copied. If all else fails, you can copy this address and paste it directly into your preferred email service.
Email Replies Blocked
In some cases, the person posting the ad on Craigslist may have responding via email disabled. In these cases, the only way to contact him is using one of the alternate contact methods provided in the ad -- usually a phone number or some other means of contact. If there aren't any alternate methods offered, there may have been an error in posting the ad.
Expired or Deleted Postings
If the posting has expired or been deleted since you first saw it, you may not be able to contact the advertiser using the reply email address. Craigslist makes interactions between an advertiser and applicant anonymous using temporary "go-between" email addresses, in order to prevent spamming and the harvesting of personal emails. When the listing or ad is remove, the temporary email address is broken and no longer works. If you suspect this is the case, use any alternative contact methods that the ad originally offered instead.