Can’t send mail with Outlook 2016 for Mac

I recently setup a new email account for a customer who is using Outlook 2016 for Mac OSX. The customer was able to setup the new account in Outlook without any issues but when he tried sending mail he received the following error

Authentication fails with error 17895

After some testing on our end we worked out that our server doesn’t offer a suitable authentication mechanism on SMTP as Outlook doesn’t support the plain mechanism.

The server we use is a Modoboa setup which uses Dovecot. in the file /etc/dovecot/conf.d/10-auth.conf there is an option called auth_mechanisms which lists the authentication mechanisms that the server will offer. Adding ‘login’ as an additional mechanism will allow Outlook to authenticate.

in a standard Modoboa setup (and probably with a standard Dovecot setup as well) the relevant section in /etc/dovecot/conf.d/10-auth.conf looks as follows:

# Space separated list of wanted authentication mechanisms:
# plain login digest-md5 cram-md5 ntlm rpa apop anonymous gssapi otp skey
# gss-spnego
# NOTE: See also disable_plaintext_auth setting.
auth_mechanisms = plain

Change this to the following

# Space separated list of wanted authentication mechanisms:
# plain login digest-md5 cram-md5 ntlm rpa apop anonymous gssapi otp skey
# gss-spnego
# NOTE: See also disable_plaintext_auth setting.
auth_mechanisms = plain login

and restart dovecot:

systemctl restart dovecot

After this you should be able to send email through your server with an Outlook for Mac client.

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