First of all, this isn't a general problem with our current version. We can't duplicate the issue with other users on the server. 

When this user creates a new message or replies using the webmail interface and uses a signature the encoded message is 1 line which when exceeding 900 some characters causes the following bounce on certaIN hosts.

Reason: Remote host said: 550 Maximum line length exceeded (see RFC 5322 2.1.1).

When looking the raw message you normally get something like this:

--8bb418ce74194413b5e550fa34c20524
Content-Type: text/html;
    charset="utf-8"Content-Transfer-Encoding: quoted-printable

<div style=3D"font-family: arial; font-size: 12px;"><div data-empty=3D"true=
"><br></div><div data-empty=3D"true"><br></div><div contenteditable=3D"fals=
e"><div>Regards</div><div>Testing Support</div></div><div data-empty=3D"tru=
e"><br></div><hr id=3D"previousmessagehr"><div><span><strong>From</strong>:=

For this user is like this:

--a2c4175598c441cc912768f42447396c
Content-Type: text/html;
    charset="utf-8"Content-Transfer-Encoding: 8bit

<div style="font-family: arial; font-size: 12px;"><div data-empty="true">Hi Christophe and Ginger,<br><br>As we discussed, Sophienwald wil etc....

- We tested with a brand new simple 1 line signature, same result.
- When no signature is used the message is encoded properly

- We checked the webmail settings which all looked standard, font face, encoding, etc...

Any idea on how to fix this? Recreating the user isn't really practical since it is a webmail user and has all data, contacts, etc stored on the server.

Edit: added version # 16.3.6768