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EDIT:
I have problems sending emails with Thunderbird. IMAP connection works without problems, but SMTP is not connecting. I have tried different settings:
No connection security (port 25), STARTTLS (port 587), but none is connecting to the server.
When I try to connect via telnet I get a timeout: telnet smtp.develman.de 25
resp. telnet smtp.develman.de 587
I have tried to connect via telnet with an online service and the connection was successfull: http://www.adminkit.net/telnet.aspx
Now I have installed wireshark to log the TCP traffic. I do not get any TCP SYN-ACK, only a TCP Retransmission.
I do not know, where the problem is. I also tried to disable clam av but without any success. My system is Archlinux.
No-one is going to be able to tell you. You've obscured the domain names involved, told us nothing about what "every try" means (not even what software is involved), told us nothing about whether this is a problem you are seeing in a MTA or an MTS, and shown us none of the actual error messages. You'll get random guesses based upon erroneous assumptions, at best. – JdeBP – 2013-12-30T19:17:36.463