Debian’s exim4 uses gnutls which has a feature that will not allow my E61i to use STARTTLS (see #438137) . Perhaps stunnel can help here …

Update: To advertise PLAIN authentication with exim4 for connections origination from localhost I used the following statement in my plaintext authentication configuration (broken into several lines for better readability):

server_advertise_condition =
 ${if or
  {
   {!eq{$tls_cipher}{}
  }
  {
   eq{$interface_address}{127.0.0.1}}
  }
  {yes}{no}
 }

That way plaintext auhentication is allowed for TLS sessions or unencrypted sessions originating from localhost.

Using stunnel and a default ssmtp configuration I can now send mails from my mobile phone.