Add a new variable smtpmail-retries

* doc/misc/smtpmail.texi (Server workarounds): Mention it (bug#34177).

* lisp/mail/smtpmail.el (smtpmail-retries): New variable.
(smtpmail-via-smtp): Use it.

doc/misc/smtpmail.texi

@@ -372,6 +372,13 @@ implement support for common requirements.
 
 @table @code
 
+@item smtpmail-retries
+@vindex smtpmail-retries
+An SMTP server may return an error code saying that there's a
+transient error (a @samp{4xx} code).  In that case, smtpmail will try
+to resend the message automatically, and the number of times it tries
+before giving up is determined by this variable, which defaults to 10.
+
 @item smtpmail-local-domain
 @vindex smtpmail-local-domain
   The variable @code{smtpmail-local-domain} controls the hostname sent

etc/NEWS

@@ -1164,7 +1164,8 @@ attempt when communicating with the SMTP server(s), the
 
 ---
 *** smtpmail will now try resending mail when getting a transient 4xx
-error message from the SMTP server.
+error message from the SMTP server.  The new 'smtpmail-retries'
+variable says how many times to retry.
 
 ** Footnote mode
 

lisp/mail/smtpmail.el

@@ -165,6 +165,13 @@ attempt."
   :type '(choice regexp (const :tag "None" nil))
   :version "27.1")
 
+(defcustom smtpmail-retries 10
+  "The number of times smtpmail will retry sending when getting transient errors.
+These are errors with a code of 4xx from the SMTP server, which
+mean \"try again\"."
+  :type 'integer
+  :version "27.1")
+
 ;; End of customizable variables.
 
 
@@ -823,7 +830,7 @@ Returns an error if the server cannot be contacted."
 		)
                ((and (numberp (car result))
                      (<= 400 (car result) 499)
-                     (< send-attempts 10))
+                     (< send-attempts smtpmail-retries))
                 (message "Got transient error code %s when sending; retrying attempt %d..."
                          (car result) send-attempts)
                 ;; Retry on getting a transient 4xx code; see