Too many recipients result in an unfolded To: header that is too long

[martinvonwittich]

I'm using swaks to validate an Exim configuration that should reject outgoing mails with more than 50 recipients:

swaks --from [email protected]  --to $(print -l recipient+{1..50}@example.com | paste -sd,) --server my.local.exim.instance --tls-on-connect -a LOGIN -au [email protected] -ap nopass

In this case the email was supposed to pass because 50 recipients is below the limit, but Exim instead froze the message with the following error:

my.local.exim.instance ~ # exigrep 1tnejz-001VBJ-19 /var/log/exim4/mainlog
+++ 1tnejz-001VBJ-19 has not completed +++
2025-02-27 15:19:55 1tnejz-001VBJ-19 <= <> R=1tnejy-001VAF-1J U=Debian-exim P=local S=15054
2025-02-27 15:19:55 1tnejz-001VBJ-19 ** [email protected] R=local_user T=dovecot_lmtp: message has lines too long for transport
2025-02-27 15:19:55 1tnejz-001VBJ-19 Frozen (delivery error message)

The problem seems to be that the To: header that swaks generated from the list of the recipients isn't folded:

To: [email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected]

It therefore has a length of 1244 characters, which is longer than RFC5322 allows:

There are two limits that this specification places on the number of
characters in a line. Each line of characters MUST be no more than
998 characters, and SHOULD be no more than 78 characters, excluding
the CRLF.

I was able to work around the issue by manually overwriting the To: header with --header 'To: [email protected]'.

The swaks version I'm using is the one provided by Debian stable (bookworm):

my.local.exim.instance ~ # lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description:    Debian GNU/Linux 12 (bookworm)
Release:        12
Codename:       bookworm
my.local.exim.instance ~ # swaks --version
swaks version 20201014.0
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