When I test the plugin with mailtrap.io everything works fine.
The configuration in user/config/plugin/email.yaml ist as follows:

enabled: true
from: [email protected]
to: [email protected]
mailer:
engine: smtp
smtp:
server: mailtrap.io
port: 2525

encryption:

user: 123546
password: 12345
sendmail:
bin: /usr/sbin/sendmail
content_type: text/html
debug: true

When I change the configuration to:

enabled: true
from: [email protected]
to: [email protected]
mailer:
engine: smtp
smtp:
server: smtp.1und1.de
port: 465
encryption: ssl
user: [email protected]
password: abcdefg
sendmail:
bin: /usr/sbin/sendmail
content_type: text/html
debug: true

I get the following error:
0 - Connection could not be established with host smtp.1und1.de [Connection timed out #110] Log data: ++ Starting Swift_SmtpTransport !! Connection could not be established with host smtp.1und1.de Connection timed out #110

I tested also with google smtpserver and all combinations with ports and encryption. Help appreciated.

Having the same problem.
Did you figure it out ?

It's a problem with the Swift email library. Check out this thread about your same issue, which suggests a Firewall issue: https://stackoverflow.com/a/14179075