If I receive a subpoena by regular mail, is that legal?

No attorney can advise you or guide you in any way to ignore a subpoena, even if improperly served.  I am going to cut and paste the Texas procedure law here so that you can read it.  It appears that if you were mailed it by regular mail that may not be proper service.  Certified mail with return receipt requested seems to be proper and would answer your question "how would they know" that you received it.  If the subpoena is from the Prosecutor's office call them.  They have great power to haul you in but if they need you they may work with you.  If it is from the defendant's attorney then contact an attorney yourself.  The proper thing to do is to make a motion to quash a subpoena for improper service and then you can explain the hardship involved.  Good luck.

Art. 24.04. [464] [527] [515] SERVICE AND RETURN OF
SUBPOENA.  (a) A subpoena is served by:
(1)  reading the subpoena in the hearing of the witness;                      
(2)  delivering a copy of the subpoena to the witness;                        
(3)  electronically transmitting a copy of the subpoena,
acknowledgment of receipt requested, to the last known electronic
address of the witness;  or
(4)  mailing a copy of the subpoena by certified mail, return
receipt requested, to the last known address of the witness unless:
(A)  the applicant for the subpoena requests in writing that
the subpoena not be served by certified mail;  or
(B)  the proceeding for which the witness is being subpoenaed
is set to begin within seven business days after the date the
subpoena would be mailed.
(b)  The officer having the subpoena shall make due return
thereof, showing the time and manner of service, if served under
Subsection (a)(1) or (2) of this article, the acknowledgment of
receipt, if served under Subsection (a)(3) of this article, or the
return receipt, if served under Subsection (a)(4) of this article. 
If the subpoena is not served, the officer shall show in his return
the cause of his failure to serve it.  If receipt of an
electronically transmitted subpoena is not acknowledged within a
reasonable time or a mailed subpoena is returned undelivered, the
officer shall use due diligence to locate and serve the witness.  If
the witness could not be found, the officer shall state the
diligence he has used to find him, and what information he has as to
the whereabouts of the witness.
(c)  A subpoena served under Subsection (a)(3) of this
article must be accompanied by notice that an acknowledgment of
receipt of the subpoena must be made in a manner enabling
verification of the person acknowledging receipt.