Workaround: "554 rejected due to spam content" sending e-mail

It sometimes happens when I reply to an e-mail from somebody who is asking about my products that the receiving mail server rejects my message with the code “554 rejected due to spam content”.

Google Apps notifies about that like this:

Delivery to the following recipient failed permanently:

     RECIPIENT EMAIL ADDRESS

Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the server for the recipient domain RECIPIENT DOMAIN by RECIPIENT MAIL SERVER. [RECIPIENT MAIL SERVER IP ADDRESS].

The error that the other server returned was:
554 rejected due to spam content

When that happens I try to send the message from another e-mail account. That works in some cases, but not always. Sometimes the second attempt fails, too, although the second message was sent from a different domain with a different mail server, whose notification looks like this:

This is the mail system at host cc-smtpout3.netcologne.de.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<RECIPIENT EMAIL ADDRESS>: host RECIPIENT MAIL SERVER[RECIPIENT MAIL SERVER IP ADDRESS] said: 554
    rejected due to spam content (in reply to end of DATA command)

Final-Recipient: rfc822; RECIPIENT EMAIL ADDRESS
Original-Recipient: rfc822;RECIPIENT EMAIL ADDRESS
Action: failed
Status: 5.0.0
Remote-MTA: dns; RECIPIENT MAIL SERVER
Diagnostic-Code: smtp; 554 rejected due to spam content

Obviously, this has nothing to do with my mail system. These are false positives where the receiving mail server finds something in my e-mails it does not like - but what? I had not attached anything and the messages were just a few lines of text.

As it turned out some security products do not like URLs in e-mails. In all cases my messages went through once I removed any links I had added. In cases where my messages did not contain any links I removed the URL to my website in the signature.