That depends on the mail server. If the mail server supports the
option, you can telnet to port 25 of the mail server, and issue an EHLO
command:
> # telnet
gmail-smtp-in.l.google.com. 25
< Trying
< 64.233.169.26...
< Connected to
gmail-smtp-in.l.google.com.
< Escape character is '^]'.
< 220
mx.google.com ESMTP 203si1683240oic.114 - gsmtp
> EHLO bob
<
250-mx.google.com at your service, [38.101.148.27]
< 250-SIZE 35882577
< 250-8BITMIME
< 250-STARTTLS
< 250-ENHANCEDSTATUSCODES
< 250-PIPELINING
< 250-CHUNKING
< 250 SMTPUTF8
From the response to the EHLO command, you can see that the enforced
size limit is 35882577 bytes (roughly 34.22mb).
Not all mail servers support this, though. If yours does not, your only
option is likely trial and error, and/or hope that if you send a message
that exceeds their size limit, the error message they return tells you
the enforced size limit.
-- Jonathan