I originally reported this bug in Launchpad against the Ubuntu package, but the maintainer asked me to report it upstream. The LP bug is here:

https://bugs.launchpad.net/ubuntu/+source/evolution/+bug/1455491

I've set severity to major as emails are lost.

This is Evolution 3.12.11 on Ubuntu Vivid with all updates installed.

Here comes the description I filed in LP:

Hi,

I have intermittent problems sending email from Evolution. Maybe every other day or so, Evolution gives the impression the email has been sent, while it hasn't been.

When this happens, this is what I see on the server side (IP/host info withheld):

May 13 08:23:28 mail postfix/smtpd[14289]: connect from unknown[IPv6:address]
May 13 08:23:28 mail postfix/smtpd[14289]: B218622E50: client=unknown[IPv6:address]
May 13 08:23:28 mail postfix/smtpd[14289]: B218622E50: client=unknown[IPv6:address]
May 13 08:23:28 mail postfix/cleanup[13287]: B218622E50: message-id=<1431498208.15596.31.camel@<withheld>>
May 13 08:23:28 mail postfix/smtpd[14289]: lost connection after DATA (64324 bytes) from unknown[IPv6:address]
May 13 08:23:28 mail postfix/smtpd[14289]: disconnect from unknown[IPv6:address]

May 15 13:14:38 mail postfix/smtpd[3632]: connect from unknown[IPv6:address]
May 15 13:14:38 mail postfix/smtpd[3632]: 21E8522E44: client=unknown[IPv6:address]
May 15 13:14:38 mail postfix/smtpd[3632]: 21E8522E44: client=unknown[IPv6:address]
May 15 13:14:38 mail postfix/cleanup[2894]: 21E8522E44: message-id=<1431688478.15596.89.camel@<withheld>>
May 15 13:14:38 mail postfix/smtpd[3632]: lost connection after DATA (64320 bytes) from unknown[IPv6:address]
May 15 13:14:38 mail postfix/smtpd[3632]: disconnect from unknown[IPv6:address]

Now, why the connection is lost might be an interesting thing to find out in and by itself, but the real concern I have is the fact that Evolution doesn't detect this error.

It seems to pretend everything is ok and copies the email to the Sent folder without indicating something went wrong.

Would it be unreasonable of me to expect Evolution to wait for a 250 first?