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 send/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

Your server needs the PHP IMAP extension installed, otherwise WP Mailster can not work at all. There is nothing that we can do about that, you need to contact your webhoster and see whether they can enable/install the PHP IMAP extension.
If that is not possible you need to use a different webhoster providing a more suitable environment.
Please note that it does not matter whether you want to use IMAP or POP3 for accessing the mailing list inbox. You need the extension in both cases, it is not specific to the IMAP protocol.

In most situations this connection errors show up when your webhoster (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 webhoster, 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 webfrontend 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 webhoster 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

Why all that?
In case someone joins the project team you would only add the user to the Developers group. Then the user would be automatically be a recipient of both the project and the company-wide mailing list.
The same thing applies if someone leaves: the user only needs to be removed form the groups and thus is automatically removed from the mailing lists that have this groups as the recipients.

So to sum up: if you run many lists and have many users that are largely managed by the admin, then it might be handy to organize users in groups and add the groups as the recipients.