Whats the number of times the letter i appears in:
Q: Does DNS A record cover emails too? In my DNS configuration Ive got
Name    Type TTL     Value
gest    A    1800    12.34.56.789

Does it mean that every emails sent to @gest.mydomain.com will be handled by a mail server (if any) on 12.34.56.789 ?

A: The sending SMTP server will attempt delivery for email messages to @gest.example.com addresses first by looking up the MX record for gest.example.com
In the absence of a MX record for gest.example.com RFC 5321  provides a fallback mechanism:

If an empty list of MXs is returned,
the address is treated as if it was associated with an implicit MX
RR, with a preference of 0, pointing to that host.

In other words: when your DNS only has the following record:
Name    Type TTL     Value
gest    A    1800    10.34.56.789

SMTP servers should treat that as if the following MX record exists:
Name    Type TTL     Value
gest    MX   0       0 gest.example.com. 
gest    A    1800    10.34.56.789

and route mail accordingly.
So yes, messages to @gest.example.com should be sent to the host with the gest.mydomain.com A record  i.e. to  10.34.56.789

user
In the text, there are forty-four "i"s.
assistant