Need ideas (suggestions) for a specific task. Strong in matters of mail servers?

Question

Computer networks

Introductory:
Actually, everything has already been invented, the question remained in the implementation. I optimize/transform a huge flow of paper documents (statements from personal bank accounts) into a flow of electronic documents - letters to e-mail. The problem of non-identification of statements (they stupidly don’t have surnames, there are only account numbers) can be solved in two ways, either by creating a catalog of account numbers with a full name, or the second option, with which I actually decided to “get confused”. I want to create a separate mailbox for each person (possibly virtual), the address of the box will correspond to the client ID, to which the client itself (more precisely, the bank) will automatically send the statements we need. Then we attach a parser to the mail, which parses the mail attachments and arranges them in the appropriate directories.
Question:
Now we use the Yandex service as a corporate mail. What mail parser would you recommend? What do you think is the best way to organize virtual mailboxes? The number of boxes is approximately 10,000 pcs. The regularity of receipt of statements is monthly, if necessary, once a year (i.e. traffic will be excessive, this is a feature of the bank). The letter contains an RTF file and a TXT file as attachments, both with the extract data. In general, I will be glad to any rational ideas that I can miss in the problem. Thank you! =)