Email sending issue with “localhost.localdomain”

Nobody cares about hops inside your infrastructure and how many localhosts are passed. But be sure to let your outgoing server say "Hello, I'm mail.domain.tld" instead of "Hello, I'm localhost.localdomain", because no halfway properly configured server will be interested to talk to unknown mailservers.

I see in your referenced headers that you use postfix(1) for processing outbound emails. Look for the variable myhostname inside your main.cf (usually located in /etc/postfix or /usr/local/etc/postfix) and fill it properly. Example: myhostname=mail.domain.tld (replace it with your mailservers name). Also be sure that smtp_banner= contains at least $myhostname like smtp_banner=$myhostname ESMTP Postfix.

See this for additional info.

can one order the elements of a finite group such that their product is equal to the first element in the list?

The limit and asymptotic analysis of $a_n^2 - n$ from $a_{n+1} = \frac{a_n}{n} + \frac{n}{a_n}$

Does $SL_2(K) \simeq SL_2(L)$ imply $K\simeq L$?

Is there an easy way to find the sign of the determinant of an orthogonal matrix?

$\int_{- \infty}^{\infty} \frac{f(x)}{1+\exp{g(x)}}dx=\int_{0}^{\infty} f(x) dx$ for $f(x)=f(-x),~g(x)=-g(-x)$ - are there other formulas like that?

Mathematical Explanation of Mathematica Summation ${\sum_{n=1}^{\infty}\frac{(2n-1)!}{(2n+2)!}\zeta(2n)}$

Deriving the closed form for $\sum_{n=-\infty}^{\infty} \tan^{-1} (an+b) $

Proof verification: $\sum_{n=1}^\infty \frac{1}{n^2}=\frac{\pi^2}{6}$

Classification of covering spaces of $\Bbb{R}\textrm{P}^2 \vee \Bbb{R}\textrm{P}^2$.

Different ways of representing a second cohomology class

Are there prime lengths in triangle with all integer sides and heights?