That depends on the content of such mails. First step is to gather those mails in raw (eml) format. I can analyze them if you send me a copy (<dietmar@proxmox.com>). Please zip them.
OK, I receive no further examples from you, so I can't create an efficient test. Anyways, I can show you how to create a custom Spamassassin test rule.
First, login on you proxmox mailgateway. The edit:
# vi /etc/mail/spamassassin/custom.cf
Code:
uri TESTRULE /smartcardgreeting.com\/\?id\=22a9665cdd1787182079651/
score TESTRULE 2
the manual page has a verbose description of the format:
# man Mail::SpamAssassin::Conf
To test the syntax of the file you can use:
# spamassassin -D --lint
To test if it work store an example mail in raw format 'tesmail.eml':