How can I make sure that most of my email invitations to recipients are not rejected by ISP spam traps?

I have set up SPF, rDNS and Dkim on my site. But the phpmailer used by default in Lime Survey doesn't add in the header lines from Dkim.

I would recommend to use a properly configured SMPT server and not the PHPmail option.

Here are a few posts in the forum that explain how to reduce the risk to run into spam traps (e.g. make sure you have a good quality list with correct email addresses, don't send out too many emails at once, avoid bad neighborhoods of your server/IP, etc.).

Sending mass emails without being suspicious of spam has become a real science and not an easy one.

You probably would have no chance to get through to our company mail server if you are not 100% perfect with the configuration of your server, the response of your server, the text of your email, etc.. Our admin has implemented extremely strict.

The big freemailers are not that strict, but they have a huge advantage: big numbers of accounts, which means if you send many emails with the same text, if you send many emails to accounts that don't exist, etc. they will spot you and mark you as spammer.

But search in the forum, you should find quite a few posts. Mazi has also written a blog post, summing up some of those posts.
www.limesurvey-consulting.com/how-to-pre...ing-treated-as-spam/

I have changed the setup to send through my postfix server, using SSL and port 465.

I sent out a test reminder to myself. The email I received included the Dkim headers, so any spam detection will be able to test that, as well as SPF and rDNS PTR domain records.

The one thing I cannot control is other nearby IP addresses from other users of the same hosting service.

Yes, the neighborhood factors is more difficult to control. Just a few rules of thumb: the cheaper the server, the easier the setup and the less the control of your provider, the higher the chance to be in a bad neighborhood or IP range (e.g. DigitalOcean and alike have become a big source of spam, from what I have heard).

Of course, the "neighborhood" problem can hit you anywhere, but chances can be minimized.

holch wrote: bad neighborhood or IP range (e.g. DigitalOcean and alike have become a big source of spam, from what I have heard).

The problem is more that the IP can change more often with cloud-like providers. The IP Pool changes more often and the Mailserver is identified via its IP for reputation. So it is often not the neighborhood (many different IP ranges with every provider these days. At least in the IPv4 universe) but the IP history. Issues with big freemailers like Yahoo or Hotmail are even occurring with a proper IP with a clean history. Sometimes users are just hitting the SPAM button on the webmail even if they subscribed to your mailing list.
To get informed about theses issues you can get your email server subscribed to a feedback loop. E.g help.yahoo.com/kb/SLN3438.html

Always use SMTP. Never use local phpmail. PHPmail uses the reputation of the IP of the webserver.

Our spam filter takes also IP ranges into account. Can't use wetransfer, because they are hosted within the same IP range of a known "spam schleuder" and our mail admin has blocked this IP neighborhood specifically.