I solved the issue.
I will post the solution here for the people that have the same
problem:
1) Need to compile mutt with SASL (and smtp and ssl, of course):
./configure --enable-pop --enable-imap --enable-smtp --with-sasl --with-ssl
2) Add the following to your .muttrc:
set smtp_authenticators="plain"
set smtp_url="smtp://user:password@xxxxxxxxxxxxxx:587/"
set ssl_starttls
set certificate_file=~/.mutt.certificates
Luis.