How to stop message headers growing too long in Outlook?

Question

We're having problems where we're getting "message has lines too long for transport" errors. We've been in contact with our email provider and they have said the issue is the message headers coming from Outlook are too long.

Part of their reply to our support request:

Every time you reply on a thread, Outlook generates a reference ID and adds it to the email header making it extremely large and eventually causing an error "message has lines too long for transport".

The only solutions they could propose was don't use Outlook, or start a new email chain regularly. If its only Outlook causing this issue, surely its a bug that could be fixed, or a setting that could be changed?

To pre-emptively answer some questions posed by the AI response:

• We're using HTML message format, not rich text
• This apply to emails both with and without attachments.
• This is the desktop version of Outlook that come with Microsoft Office Home and Business 2019

Answer 1

Hi

Appreciate this is quite an old thread, but we're also facing this same issue: when using Outlook to reply to a particularly long thread, all the references in the References: header are on one line:

References: <...> <...> <...> ...

exceeding the 998 character limit for a line mandated in RFC 5322. It gets bounced by the mail server with "message has lines too long for transport". When using a different mail client (thunderbird or roundcube), each reference is on a separate line:

References: <...> 
<...> 
<...>

This is unaffected by setting wrap text at character .

Cheers Toby

Answer 2

Hi Robert

Thanks so much for following up, happy that's it's not been affecting you.

Cheers

Toby