Mosaico mailings not sending
Based on your comments, I believe I see the issue.

CiviMail requires bounce processing to be enabled to work properly.  The documentation might want to emphasize this point - but your error is actually mentioned in the VERP Support section.

VERP is a standard by which you send each email from a different "From" address - so when a server responds that the recipient address is no good, you know exactly which email triggered the problem.  CiviMail determines the VERP address in part by the bounce processing address.  With no bounce processing address, you get the "invalid sender address" either.  You can solve this by configuring a bounce processing address.

EDIT

logs:

Jul  6 11:24:11 srv1 postfix/pickup[15216]: B363D8F4B73: uid=10006 from=&lt;b.2999.15367.c7a49eeef87f384f@&gt;
Jul  6 11:24:11 srv1 postfix/cleanup[19523]: B363D8F4B73: message-id=&lt;20180706092411.B363D8F4B73@shost.tld&gt;
Jul  6 11:24:11 srv1 postfix/qmgr[15217]: B363D8F4B73: from=&lt;b.2999.15367.c7a49eeef87f384f@&gt;, size=89808, nrcpt=1 (queue active)
Jul  6 11:24:11 srv1 postfix/smtp[19542]: B363D8F4B73: to="&lt;emailaddress&gt;", relay=aspmx.l.google.com[2a00:1450:4013:c04::1b]:25, delay=0.09, delays=0.01/0/0.07/0.01, dsn=5.1.2, status=bounced (host aspmx.l.google.com[2a00:1450:4013:c04::1b] said: 553-5.1.2 The sender address &lt;b.2999.15367.c7a49eeef87f384f@&gt; is not a valid 553 5.1.2 RFC-5321 address. u42-v6si5509030edm.404 - gsmtp (in reply to MAIL FROM command))