I set up my mail account on thunderbird.Mails are being downloaded from server.However when i try to send a mail using thunderbird i get the following message:
Sending of message failed.
The message could not be sent using SMTP server mail.pterzis.gr for an unknown reason. Please verify that your SMTP server settings are correct and try again, or contact your network administrator.

I have to mention when i log into roundcube web inteface, i can send mail without any problem

thanks

What does your mail logs say

/var/log/mail.log

Jan 26 06:01:00 maps imapd: TIMEOUT, user=panterz, ip=[::ffff:79.131.185.99], headers=0, body=0, rcvd=62, sent=396, time=1802
Jan 26 06:07:06 maps imapd: LOGOUT, user=panterz, ip=[::ffff:79.131.185.99], headers=7040, body=57046, rcvd=954, sent=74186, time=2181
Jan 26 06:07:16 maps imapd: Connection, ip=[::ffff:79.131.185.99]
Jan 26 06:07:16 maps imapd: Connection, ip=[::ffff:79.131.185.99]
Jan 26 06:07:16 maps imapd: Disconnected, ip=[::ffff:79.131.185.99], time=0
Jan 26 06:07:16 maps imapd: Disconnected, ip=[::ffff:79.131.185.99], time=0
Jan 26 06:07:20 maps imapd: Connection, ip=[::ffff:79.131.185.99]
Jan 26 06:07:20 maps imapd: Disconnected, ip=[::ffff:79.131.185.99], time=0
Jan 26 06:07:36 maps imapd: Connection, ip=[::ffff:79.131.185.99]
Jan 26 06:07:36 maps imapd: Disconnected, ip=[::ffff:79.131.185.99], time=0
Jan 26 06:08:05 maps imapd: Connection, ip=[::ffff:79.131.185.99]
Jan 26 06:08:05 maps imapd: LOGIN, user=panterz, ip=[::ffff:79.131.185.99], port=[55827], protocol=IMAP
Jan 26 06:08:39 maps postfix/smtpd[5896]: connect from mail-pz0-f41.google.com[209.85.210.41]
Jan 26 06:08:40 maps postfix/smtpd[5896]: setting up TLS connection from mail-pz0-f41.google.com[209.85.210.41]
Jan 26 06:08:40 maps postfix/smtpd[5896]: Anonymous TLS connection established from mail-pz0-f41.google.com[209.85.210.41]: TLSv1 with cipher RC4-SHA (128/12$
Jan 26 06:08:40 maps postfix/trivial-rewrite[5899]: warning: database /etc/postfix/transport.db is older than source file /etc/postfix/transport
Jan 26 06:08:40 maps postfix/trivial-rewrite[5899]: warning: do not list domain pterzis.gr in BOTH mydestination and virtual_alias_domains
Jan 26 06:08:40 maps postfix/smtpd[5896]: C79453A136: client=mail-pz0-f41.google.com[209.85.210.41]
Jan 26 06:08:41 maps postfix/cleanup[5900]: C79453A136: message-id=<CABz5BQkdPniYYPgpUSZiVpFGaPK9AFgBvaDRj6jOcJ6SL_UzdQ@mail.gmail.com>
Jan 26 06:08:41 maps postfix/qmgr[2551]: C79453A136: from=<thanterz@gmail.com>, size=1546, nrcpt=1 (queue active)
Jan 26 06:08:41 maps postfix/pickup[5722]: 5AF503A13D: uid=5001 from=<thanterz@gmail.com>
Jan 26 06:08:41 maps postfix/cleanup[5900]: 5AF503A13D: message-id=<CABz5BQkdPniYYPgpUSZiVpFGaPK9AFgBvaDRj6jOcJ6SL_UzdQ@mail.gmail.com>
Jan 26 06:08:41 maps postfix/pipe[5901]: C79453A136: to=<panterz@mail.pterzis.gr>, orig_to=<panterz@pterzis.gr>, relay=spamassassin, delay=0.72, delays=0.48/$
Jan 26 06:08:41 maps postfix/qmgr[2551]: C79453A136: removed
Jan 26 06:08:41 maps postfix/qmgr[2551]: 5AF503A13D: from=<thanterz@gmail.com>, size=1877, nrcpt=1 (queue active)
Jan 26 06:08:41 maps postfix/local[5905]: 5AF503A13D: to=<panterz@mail.pterzis.gr>, relay=local, delay=0.01, delays=0/0.01/0/0, dsn=2.0.0, status=sent (deliv$
Jan 26 06:08:41 maps postfix/qmgr[2551]: 5AF503A13D: removed
Jan 26 06:08:45 maps imapd: Connection, ip=[::ffff:79.131.185.99]
Jan 26 06:08:45 maps imapd: LOGIN, user=panterz, ip=[::ffff:79.131.185.99], port=[55828], protocol=IMAP
Jan 26 06:09:05 maps imapd: LOGOUT, user=panterz, ip=[::ffff:79.131.185.99], headers=0, body=0, rcvd=762, sent=817, time=20
Jan 26 06:09:05 maps imapd: LOGOUT, user=panterz, ip=[::ffff:79.131.185.99], headers=266, body=3892, rcvd=752, sent=6834, time=60
Jan 26 06:09:11 maps postfix/smtpd[5896]: disconnect from mail-pz0-f41.google.com[209.85.210.41]
Jan 26 06:12:31 maps postfix/anvil[5898]: statistics: max connection rate 1/60s for (smtp:209.85.210.41) at Jan 26 06:08:39
Jan 26 06:12:31 maps postfix/anvil[5898]: statistics: max connection count 1 for (smtp:209.85.210.41) at Jan 26 06:08:39
Jan 26 06:12:31 maps postfix/anvil[5898]: statistics: max cache size 1 at Jan 26 06:08:39

warning: do not list domain pterzis.gr in BOTH mydestination and virtual_alias_domains

I don't know if this is your problem, but I would certainly take a look as to why you are getting a warning in the log.

--
Travis