Just lost an hour or so for a simple task: Configuring sendmail to use smart_host with authentication on CentOS7.

TL;DR: "AUTH=client, available mechanisms do not fulfill requirements" just needed a "yum install cyrus-sasl-plain"

I started with configuring sendmail to use smart_host with auth credentials:

1. Add authorization informations to /etc/mail/authinfo file:

AuthInfo:your-smart-host.tld "U:USERNAME" "P:PASSWORD"

2. Add following lines to /etc/mail/sendmail.mc:

define(`SMART_HOST', `your-smart-host.tld')  
FEATURE(`authinfo')

3. Update sendmail.cf and access.db files:

# cd /etc/mail
# make
# systemctl sendmail restart

But maillog showed temporary failures AUTH=client, available mechanisms do not fulfill requirements.

I checked the servers supported authentication methods with:

# echo -e "ehlo me\nquit" | openssl s_client -starttls smtp -crlf -quiet -connect your-smart-host.tld:25 2>/dev/null | grep AUTH
250-AUTH LOGIN PLAIN

Kept debugging, logging and searching google. Found a forum post where someone solved it by installing cyrus-sasl*.

Well it turned out that only cyrus-sasl-plain was missing and the solution was a simple

yum install cyrus-sasl-plain