WP Mailster

When you want to use WP Mailster you don't need to browse to a website, login and do something to send the message there - you just use your favorite mail client. Simply write an email to the mailing list's address - and nothing else. So use Gmail, Outlook, Thunderbird, a Webmailer, any way you like - just send it to the mailing list address you have setup in WP Mailster.

WP Mailster is a part of WordPress which is a PHP based web application. That means: it can not act/run without being triggered and it can not run forever when triggered. This is a technical limitation coming from PHP, not from WP Mailster or WordPress. Triggering means that somebody accesses the site. During the page load WP Mailster is handling the jobs to do (mail retrieving/sending). Thus mails can only be sent/retrieved when somebody is browsing your site, otherwise the delivery is delayed or never done. As your site might not be browsed every few minutes 24×7 we recommend you to use a cronjob that opens the site periodically. We have a guide on our website on how to set that up.

The send errors are messages your email server is giving back to WP Mailster basically saying "I will not forward this message". Then WP Mailster sending for some time but eventually stops which is what you see happening. The cause can be a lot of things, e.g. hitting send limits (per hour/day) or sending email with content that the server does not like. You need to find out what your email servers are telling WP Mailster. Please follow our troubleshooting guide on our site.

You might get an error message like this when checking the connection settings: “Certificate failure for [server] Server name does not match certificate” or “Certificate failure for [server] unable to get local issuer certificate”. This is often the case when you use your own mailing server (with a self-signed certificate). To get a connection established, you need to deactivate the automated certificate check. Do this by adding the following line in the special parameter box: /novalidate-cert

WP Mailster requires PHP 7.4 or newer for all mailbox backends. The legacy PHP IMAP backend needs the PHP IMAP extension for both IMAP and POP3. Ask your web host to enable it if you use this backend. The bundled Webklex backend can access IMAP mailboxes without the PHP IMAP extension. It requires PHP 7.4 or newer, the OpenSSL, mbstring, iconv and fileinfo extensions, and JSON support. WP Mailster 1.9.0 supports PHP 8.4 and 8.5 using this Webklex backend for IMAP mailboxes. The PHP IMAP extension was removed from the bundled PHP distribution in PHP 8.4 and moved to PECL; Webklex removes the need to install it for IMAP access. POP3 mailboxes still require the legacy backend and the PHP IMAP extension. Automatic backend selection prefers the legacy PHP IMAP backend when available. Google and Microsoft OAuth inboxes use Webklex.

Create or edit an inbox server in WP Mailster, select OAuth 2.0 and the provider, and enter your OAuth application's client ID and client secret. Register the exact OAuth Redirect URI shown in the server settings with your application provider. Select that server in the mailing list's mailbox settings, save the list, then use the provider's connect button to authorize the mailbox. Run the inbox connection check after authorization; successful authorization alone does not confirm that the mailbox permits IMAP access. Tokens are refreshed during regular mailbox retrieval. If access is revoked, reconnect the mailbox. OAuth support applies to incoming IMAP mail; configure outgoing SMTP separately.

In most situations this connection errors show up when your web hoster (where WordPress/WP Mailster is installed) is blocking outgoing connections such as connection attempts to 3rd party email servers. This is done to avoid spam sending. Please contact your web hoster, so they configure their firewall to allow you to send emails.

Your issue is very likely coming from a large email causing an PHP out of memory error. This is resulting from the fact that every PHP environment has a memory limit. This limit is hit when the email is tried to be retrieved from the mail server. Please remove this email from the mailing list in question. Either through a mail web-frontend or with the “Delete first email in inbox” tool the “Tools” section in the mailing list’s settings. In order to prevent have this happen in the future:

• Raise the PHP memory limit. Your web hoster can help you with that.
• Introduce a maximum email size for WP Mailster. You can find this setting in the “List behaviour” tab of the mailing lists settings. If larger emails are in the inbox, WP Mailster will not try to process them but simply delete it and tell the sender what happened.

A mailing list is a set of recipients that should all get the messages send to the mailing list’s email address. A group is a way of organizing users. Think of it like a bucket of users. It comes in handy if you want certain users be recipients of multiple mailing lists. Then you would not add the users directly to the recipients of a mailing list, but rather add a group. An example would be that you have three groups: Management, Project Coordinators and Developers. If you want to run a company-wide mailing list (e.g. all@example.com) and a project mailing list (e.g. project.import@example.com) then you would add

• all three groups (Management, Project Coordinators and Developers) as recipients of the company-wide mailing list
• only the Project Coordinators and Developers groups as recipients of the project mailing list

You can report security bugs through the Patchstack Vulnerability Disclosure Program. The Patchstack team help validate, triage and handle any security vulnerabilities. Report a security vulnerability.