Question about '50-user' setting

[Damian]

> What does “force MTA conversion to 7-bit (e.g. before DKIM signing)” mean?

You are probably referring to

> smtpd_discard_ehlo_keywords => ['8BITMIME']

If an MTA has 8BITMIME [1] mail to deliver to a next hop (e.g. an amavis instance), and this next hop does not announce being 
capable of handling 8BITMIME mail, the current MTA has two options, per RFC:

> A client SMTP has two options in this case: first, it may implement a
> gateway transformation to convert the message into valid 7bit MIME,                                                                                                                                                                                                                                   
> or second, or may treat this as a permanent error and handle it in
> the usual manner for delivery failures. 

So with this setting, amavis hopes for the first option, e.g. Postfix may perform such conversion per [2].

[1] https://www.rfc-editor.org/rfc/rfc1652
[2] http://www.postfix.org/postconf.5.html#disable_mime_output_conversion