On trying to send e-mail using an IMAP/SMTP server with SSL/STARTTLS on a non-standard port (587), claws 3.7.10 on Debian Wheezy reports unspecified problems with sending the message. Since claws is the only program that has any trouble whatever with DNS lookup, I thought the deb-maintainer would not be the ideal recipient of this bugreport.

To reproduce, 

1) have an IMAP account at TU Vienna
2) configure outgoing mail as follows:
  
  server mail.student.tuwien.ac.at
  SMTP-AUTH using TLS/STARTTLS Port 587

3) Try sending email

Result: Claws reports an unspecified error. The log says "Error occurred while sending the message"

Expected result: A sent message.

Additional information: 

* I first suspected SSL to cause trouble, so I did an strace to see what claws was doing. I attached the only thing that seemed connected to message sending below.

* gnutls-cli worked as expected (i.e. worked)

* Using the IP instead of the hostname, sending over the same server with otherwise unchanged configuration works. I attached a tcpdump of udp/53 below, also.

* Changing /back/ from the IP to the hostname doesn't reintroduce the error, but rebooting and then trying does. I am unsure why.

* Other mail programs where able to send messages using the server when claws was not.

* Polling the IMAP folders always worked.

Strace:
01:06:54.022056 sendmsg(10, {msg_name(0)=NULL, msg_iov(2)=[{"l\1\1\1\214\0\0\0\r\0\0\0\177\0\0\0\1\1o\0\25\0\0\0/org/fre"..., 144}, {"\207\0\0\0type='signal',sender='org.fr"..., 140}], msg_controllen=0, msg_flags=0}, MSG_NOSIGNAL) = 284
01:06:54.022363 sendmsg(10, {msg_name(0)=NULL, msg_iov(2)=[{"l\1\1\1\256\0\0\0\16\0\0\0\177\0\0\0\1\1o\0\25\0\0\0/org/fre"..., 144}, {"\251\0\0\0type='signal',sender='org.fr"..., 174}], msg_controllen=0, msg_flags=0}, MSG_NOSIGNAL) = 318
01:06:54.022737 sendmsg(10, {msg_name(0)=NULL, msg_iov(2)=[{"l\1\1\1\214\0\0\0\17\0\0\0\177\0\0\0\1\1o\0\25\0\0\0/org/fre"..., 144}, {"\207\0\0\0type='signal',sender='org.fr"..., 140}], msg_controllen=0, msg_flags=0}, MSG_NOSIGNAL) = 284
01:06:54.023041 sendmsg(10, {msg_name(0)=NULL, msg_iov(2)=[{"l\1\1\1\256\0\0\0\20\0\0\0\177\0\0\0\1\1o\0\25\0\0\0/org/fre"..., 144}, {"\251\0\0\0type='signal',sender='org.fr"..., 174}], msg_controllen=0, msg_flags=0}, MSG_NOSIGNAL) = 318
01:06:54.023407 sendmsg(10, {msg_name(0)=NULL, msg_iov(2)=[{"l\1\0\1#\0\0\0\21\0\0\0\177\0\0\0\1\1o\0\25\0\0\0/org/fre"..., 144}, {"\36\0\0\0org.freedesktop.NetworkManag"..., 35}], msg_controllen=0, msg_flags=0}, MSG_NOSIGNAL) = 179
01:06:54.023791 sendmsg(10, {msg_name(0)=NULL, msg_iov(2)=[{"l\1\0\1\0\0\0\0\22\0\0\0\206\0\0\0\1\1o\0\37\0\0\0/org/fre"..., 152}, {"", 0}], msg_controllen=0, msg_flags=0}, MSG_NOSIGNAL) = 152

tcpdump:
01:10:13.802405 IP (tos 0x0, ttl 64, id 30526, offset 0, flags [DF], proto UDP (17), length 71)    192.168.1.215.52676 > 192.168.1.1.53: [bad udp cksum 295c!] 7549+ A? mail.student.tuwien.ac.at. (43)