aioSMTPD accepts invalid RCPT TO:Postmaster without angle brackets instead of rejecting it

### Description
aioSMTPD accepts the invalid recipient syntax “RCPT TO:Postmaster” (missing angle brackets) and returns 250 OK, instead of rejecting it as a bad recipient address. The special Postmaster case that MUST be supported requires angle brackets; without them, the command should be rejected.

### Affected Version
1.4.6

### Steps to Reproduce
1. Start the aioSMTPD server on 127.0.0.1:8034.
2. Connect to the server (e.g., telnet 127.0.0.1 8034).
3. Issue the commands in order:
- EHLO client.example
- MAIL FROM:
- RCPT TO:Postmaster
4. Observe the server’s response to the RCPT command.

### Buggy Behavior
The server responds “250 OK” to “RCPT TO:Postmaster”, accepting the recipient without angle brackets.

### Expected Behavior
The server should reject “RCPT TO:Postmaster” as invalid recipient syntax and return a 501 5.1.7-style error, only accepting the special Postmaster case when enclosed in angle brackets (“RCPT TO:”).
As per RFC 5321 [4.5.1]: "[4.5.1] The requirement to accept mail for postmaster implies that RCPT commands that specify a mailbox for postmaster at any of the domains for which the SMTP server provides mail service, as well as the special case of \"RCPT TO:\" (with no domain specification), MUST be supported."