Topic: How high is the risk that bulk e-mails sent via ConfTool are identified as spam?

Last year we used special software for sending bulk e-mails, often labeled as spam, even though we did not send many.
We have yet to find a solution to this problem.

Can you explain how ConfTool sends out bulk e-mails?
Do you have any statistics for the e-mails that are labeled as spam, and how do you avoid such problems?
What can we do to avoid such problems?

(A German version of this article is available.)

Unfortunately, it is impossible to completely eliminate the risk of spam, as every spam filter employs unique rules that are not standardized.
Nonetheless, we have taken every possible action to minimize the likelihood.

Our most crucial measures include:

With ConfTool, all e-mails, including bulk e-mails, are sent as individual e-mails, not as CCs or BCCs.
Mails are sent in plain text format as default.
(You can use HTML formatted mails for bulk mailings in ConfTool Pro, but this increases the risk of mails to be classified as spam).
Our mail servers are accurately set up and do the full SMTP protocol (sounds self-evident, but it isn't).
We can provide a sender address for each event that matches the domain name of the mail server (and passes DNS and SPF tests, see below).
Example: conferenceXYZ@conftool.pro as alias for conferenceXYZ@yourdomain.edu
We provide SPF records (see below) and DKIM signatures for all mails sent from our mail servers if you use the provided alias as FROM address.
Our servers try to submit all mails via secure connections (SMTPS) if supported by the receiving mail server.
The mail servers use extended validation SSL certificates by Comodo.
Our mail servers are not listed on any (DNSBL) black lists (see below).
Mails are (usually) sent out without attachments (see: Sending bulk e-mails with (big) attachments).

You can run DNSBL (black list) tests here:
https://www.dnsbl.info/ and
https://multirbl.valli.org/lookup/ and
https://ipremoval.sms.symantec.com/ and
https://www.msp-it.de/webdienste/dnsbl-test/ (German)

You find more information on SPF here:
https://en.wikipedia.org/wiki/Sender_Policy_Framework and
https://www.openspf.org/
You can check if your mail domain uses SPF here:
https://www.openspf.org/Why

You can find more information on DKIM at:
https://en.wikipedia.org/wiki/DomainKeys_Identified_Mail
and
https://www.dkim.org/