Is this person receiving error messages? You might check to see if your email provider is restricting access due to attachments . Thunderbird would not deny receipt of an email.

Hello, I am not receiving messages from a particular sender. How can I unblock this ? This is an important customer for me and I need to receive emails from him. Greetings, Pawel Siedlecki

Have you checked your Spam/Bulk/Junk folder? If it's a POP account, you'll have to check webmail, and if senders' mail is mistakenly sent to Spam, unblock or whitelist them through webmail controls.