What is the upper limit allowed for smtp_line_length_limit?

[Curtis]

Yes, I understand that setting smtp_line_length_limit above 998 is not 
recommend.

I agree that for most Postfix installations, where mail is relayed to 
the outside world, this recommendation is your best bet for email 
deliverability because lines longer than 1000 violates RFC 5321.  I 
totally agree with this if your Postfix installation is sending outbound 
email.

I manage several gateway mail servers that only accept inbound email and 
deliver to internal servers.  Lately we've been seeing more and more 
DKIM signed messages.  Most make it though these gateway servers without 
issue.  However, we've recently seen some messages fail DKIM checks by 
the internal mail servers because some Microsoft mail clients/servers 
are sending messages all on one line, but after they come through our 
gateway servers, these messages are broken up into several lines 
(according to the smtp_line_length_limit setting).

I do not have any control over what the senders use to send these 
inbound messages and so I would like to allow these messages through 
(even if they violate RFC specs) without alteration. Since I have no 
idea how long the messages might be, I am experimenting with setting the 
smtp_line_length_limit to 9999999 (almost 10MB).   I had wondered if I 
might run into an internal upper limit that Postfix might not allow, but 
it seemed to accept 9999999 without complaint.  This has allowed these 
DKIM signed messages to come through without issue.

My questions are:

1. Considering that we're not using this setup for outbound mail, could 
there be some downsides to setting the smtp_line_length_limit this high?

2. Is there a limit to how high this setting can be set?  Would setting 
it to something like 50MB cause issues for Postfix in terms of stability?

Thanks,

Curtis