SMTPUTF8 error with Postfix

Hey, I have a very simple Postfix install with support for EAI. When I send an email manually it looks like this:

```
$ telnet example.网站 25
Trying 2a01:dead:beef::1...
Connected to example.网站.
Escape character is '^]'.
220 example.xn--5tzm5g ESMTP Postfix (Debian/GNU)
EHLO localhost
250-example.xn--5tzm5g
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-DSN
250-SMTPUTF8
250 CHUNKING
MAIL FROM: SMTPUTF8
250 2.1.0 Ok
RCPT TO:<管理者@example.xn--5tzm5g>
250 2.1.5 Ok
DATA
354 End data with .
Date: Tue, 20 Feb 2024 17:52:59 +0100
To: 管理者@example.网站
From: user@localhost
Subject: testmail
X-Mailer: Hand-written Telnet session

Hey this is a test mail

.
250 2.0.0 Ok: queued as 8D19020BD3
QUIT
221 2.0.0 Bye
Connection closed by foreign host.
```

Can I do something similar with swaks? Currently it seems not to advertise SMTPUTF8 support in MAIL FROM message
```
$ swaks
To: 管理者@example.xn--5tzm5g
=== Trying example.xn--5tzm5g:25...
=== Connected to example.xn--5tzm5g.
<- 220 example.xn--5tzm5g ESMTP Postfix (Debian/GNU)
-> EHLO localhost
<- 250-example.xn--5tzm5g
<- 250-PIPELINING
<- 250-SIZE 10240000
<- 250-VRFY
<- 250-ETRN
<- 250-STARTTLS
<- 250-ENHANCEDSTATUSCODES
<- 250-8BITMIME
<- 250-DSN
<- 250-SMTPUTF8
<- 250 CHUNKING
-> MAIL FROM:
<- 250 2.1.0 Ok
-> RCPT TO:<管理者@example.xn--5tzm5g>
<** 553 5.6.7 Must declare SMTPUTF8 to send unicode address
-> QUIT
<- 221 2.0.0 Bye
=== Connection closed with remote host.
```

Start by reproducing the SMTPUTF8 failure shown in the issue with swaks against a server advertising SMTPUTF8. Trace how swaks builds the MAIL FROM command and make the completed behavior declare SMTPUTF8 when needed, while preserving the existing SMTP exchange.

Written by the indexing model from the issue text.