Base locus of divisors on blowings up of the projective space Let $X$ be the blowing-up of $\mathbb{P}^n$ in $r$ points in general position.
Let $\{H,E_1,...,E_r\}$ be the standard basis of $Pic(X)$. Further let $D=dH-\sum m_j E_j$, be an effective divisor, with $m_j \in \mathbb{Z}$.
Is it true that if $m_1\geq 0$ then $E_1 \not\subseteq Bs(|D|)$?
If we suppose all the $m_i$'s are non negative, this corresponds to say that
$dim |dH-m_1E_i...-m_rE_r|>dim |dH-(m_1+1)E_1-...-m_rE_r|$.
In other words this means that if we consider all the hypersurfaces of $\mathbb{P}^n$ of degree $d$ passing through $r$ general points $\{p_1,...,p_r\}$ with multiplicities $m_1,...,m_r$, then they do not all pass through $p_1$ with multiplicity $m_1+1$.
My intution brings me to say this is obvious but I cannot prove it.
In fact everything is trivial for nonspecial linear systems, but, a priori, special linear systems may be a problem.
Note that the generality of the points is necessary: for example you can simply take $\mathbb{P}^2$ blown up in 3 collinear points and note that $E_1 \subseteq Bs(|H-E_2-E_3|)$.
 A: It seems to me that my question is easier than SHGH conjecture. 
I think that prop. 2.3 in "Weakly defective varieties" by Chiantini-Ciliberto is a positive answer to my question.
A: In the same vein as in a comment above, I'll comment when the base locus gets multiplicity. In such a case, this is equivalent to the Harbourne-Gimigliano-Hirschowitz conjecture which more or less reads as follows for $\mathbb P^2$:
a linear system $L$ is special iff it contains a multiple $(-1)$-curve in its base locus.
Where $L=\mathcal O(dH-\sum m_ip_i)$ and special means $h^1(L)\neq 0$. Here is interesting point is that all the points are in general position and even so, they might very well generate a special linear system. Besides the part that reads "multiple" is important. Here is an example where one has a $(-1)$-curve in the base locus and the linear system is NOT special,
all the plane cubics having two double points at $p_1, p_2$
Here, such cubics are all of the form $QL$ where $L$ is the line  $p_1p_2$. Now, $L$ is a $(-1)$-curve and it is in the base locus. However the linear system is not special.
By the way, apparently example to keep in mind of what goes wrong with these special linear systems is of the following type.
Plane quartics having 5 double points.
The space of quartics has dimension 14 and 5 double points impose 15 conditions, therefore this linear system is expected to be empty. However take a conic $Q$ passing through those points and double it $Q^2$ (here the multiple of the curve showed up). This is quartic having 5 double points as we asked. This says that neither the general position is important here nor the system is empty. The conjecture claims that that multiple $(-1)$-curve characterizes all the special linear systems!
S. Yang solved this conjecture for $\mathbb P^2$ and multiplicity of the points less than 7.
Alexander-Hirschowitz solved the case for $\mathbb P^n$ and multiplicity 2. Here is a reference
