Hello, Harster. Sender/recipient e-mail masking is commonly used to display sender/recipient name instead of real e-mail address for better user experience. YOUR real e-mail address can be found in e-mail headers (CTRL+U on a selected message) though, but I don't think it's needed in your case. Looks like spammer uses this masking functionality to mask your address with @somekindofnonsense. If you want to block these e-mails, you must find the SENDER'S e-mail address (CTRL + U can be used for this too. Search form the "From:" field) and crate a filter to delete them automatically (or move them to another folder etc.) using (Tools > Message Filters > New) and enter this email in criteria (From > contains > {spammer@email}) alongside with other needed fields. For this approach to work sender should use the same e-mail address every time, which can be not the case. Another approach to this problem, if you have configured your Gmail's account in Thunderbird as IMAP (to check - right-click on your e-mail account in Thunderbird > Settings > Server Settings > Server Type:), would be to tech Gmail to recognize these email as scam by selecting "Report spam" for every of them for a couple of days when connected to your Gmail inbox through web browser (like Google Chrome, etc.). This way in the future these type of mail should be put directly to the Spam folder in Thunderbird too.

Thanks very much gacuxz! I very much appreciate your help. I have not yet taught Gmail to recognize spam, as in your last paragraph, but I will do that soon. Thanks again.